Sometimes it's difficult for me as a learner to remember the negative word as a new word with prefix. For example, the negative of pleasant is unpleasant.

Can I safely use not instead? I'm not intending to not learn new words, but I want to prevent something awkward happening if, for example, I want to say the negative word for honest and mistakenly I say unhonest instead of dishonest.

Is it always safe to use 'not' for words I want to negate, or there are particular words where it isn't safe?


6 Answers 6


Often the "un" form is stronger than the not form.

               happy      neutral       sad
more happy <----------|------|------|--------->less happy

Unhappy is often roughly the same as "sad", but "not happy" could include "neutral" and "sad"

And there are things to watch out for:

You look not happy.

You don't look happy.

Only the second of these is idiomatic.

While you can be understood with "not ...", developing fluency means being confident with the un- in- dis- and other negating prefixes.

In short, it is unsafe to default to 'not' as your meaning could easily be misconstrued. It's also not a good habit to form for your long term progress.

  • Yeah, +1, even though it's not a prefix form, consider the idiom, "not bad", which is often more emphatic than just, "good".
    – kojiro
    Commented Aug 29, 2021 at 17:46
  • 1
    The advice here is excellent, but i don't think this actually answers the question
    – Dúthomhas
    Commented Aug 29, 2021 at 17:53
  • 5
    "not happy" is also often used to annoyance, mild anger, disappointment Commented Aug 30, 2021 at 3:25
  • @kojiro Indeed. Or even "not good", which is pretty commonly used to emphasize a particularly bad situation.
    – J...
    Commented Aug 30, 2021 at 16:00
  • Good answer. In other words, to say a proposition is not true is logically different from saying the opposite of the proposition is true. And good point about idioms as well.
    – jschmitter
    Commented Aug 30, 2021 at 17:30

There are some words that can take several different negative prefixes, and the different compound words have different meanings. Typically, one is the simple antonym and the others have narrower meanings. Non- typically means “something other than,” dis- “the opposite of, in a bad way,” a- “not having,” and anti- “opposed to,” but un- can be particularly flexible. Which words allow which prefixes is completely arbitrary. Some examples include:

Non-moral (not a moral issue), versus immoral (worthy of moral condemnation) versus amoral (a person who does not believe in right or wrong, an action taken without regard for morality, or something that lacks any moral dimension)

Uninterested (not interested) versus disinterested (having no conflict of interest). Watch out, though: some native speakers use these as synonyms, and others consider that an error

Nonfunctional (not functional) versus dysfunctional (functions in an awful or abusive way)—but note that dysfunction is a noun that means a failure to function, malfunction is either a noun or verb that means to function incorrectly, and there is no such word as *nonfunction or *malfunctional. (But there is malfunctioning.)

Nonsexual (content that is not sexual), versus asexual (an organism without sexual differentiation, or a person without a sex drive) versus antisexual (opposed to sex)

Unconcerted (not concerted, unplanned, not an organized effort) versus disconcerted (confused, perturbed)

Non-sensitive (not painful to touch) versus insensitive (not sensitive to other people’s feelings, callous)

Sometimes the base word and a compound that reverses it have evolved so much that they no longer are even opposites. The words ingenuous, disingenuous and ingenue have diverged to the point that they no longer have anything to do with each other. They all derive from the same word meaning “honest, plain-speaking, high-minded” (or literally, possessing “inborn” virtue), which came to mean naïve and honest to a fault, but ingenuous got mixed up with ingenious, an ingenue is an innocent young woman, and disingenuous means pretending not to understand or know things that you really do. An example is when antisemites disingenuously pretend that the word antisemitic means something other than hating Jews and act like it refers to the Semitic language group. The people who say that know better and are trolling.

That was also an example of how in- sometimes means the-opposite-of, but often is a fossilized Latin preposition instead. An ingenious person is clever and inventive, not the opposite of a genius.

Similarly, unbarred means not blocked or prohibited, but disbarred comes from a different root and means that someone was banned from the legal profession for unethical conduct. However, an unfrocked Catholic is a (rarely-used word for a) defrocked priest, not a layperson, and unrobed is a rarely-used synonym for disrobed.

In front of some ethnonyms, such as American, Christian, Muslim or so on, you can add negative prefixes with very different connotations. Non-Jewish is a neutral term meaning “not Jewish,” Anti-Muslim is a disparaging term meaning “bigoted against Muslims,” and un-Christian means “hypocritically antithetical to the values of Christianity.” The exact connotations vary.

Or to get really obscure, anti-establishment means trying to topple the powers that be, non-establishment means holding different views, but disestablishment has a very narrow meaning: abolishing what the U.S. Constitution calls a “law regarding an establishment of religion,” and revoking the special legal status of a state religion. There’s even a word anti­dis­est­ab­lish­ment­ar­ian­ism, the movement of people opposed to disestablishing the state religion, although most appearances of it are in lists of the longest English words.

  • So, we will be hearing anti-establishmentarians soon with regard to Afghanistan?
    – Lambie
    Commented Aug 28, 2021 at 21:12
  • 1
    @Lambie I suppose antidisestablishmentarian could be a very polite way to describe the Taliban.
    – Davislor
    Commented Aug 28, 2021 at 21:27
  • The word “amoral” is used less often to describe a person or their beliefs, and more often to describe an action or decision—that is, that this is an action or decision that isn’t a moral question, that the various options are equally moral (or equally amoral, or equally immoral) and the choice has to be made, or judged, on other grounds.
    – KRyan
    Commented Aug 29, 2021 at 3:49
  • Also, it might be worth noting that antidisestablishmentarianism was originally opposed specifically to the disestablishment of the Church of England.
    – KRyan
    Commented Aug 29, 2021 at 3:52
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    @Bobson I guess insensitive versus non-sensitive would be a genuine example of that, but it’s much more common to see words from Latin where in- is a fossilized preposition.
    – Davislor
    Commented Aug 30, 2021 at 21:59

There are some cases where not X and unX aren't equivalent, for example unbelievable and not believable:

This person is unbelievable. (amazing, astonishing)

This person is not believable. (not trustworthy)

at least one case where X and inX share the same meaning (from @abligh's comment):


same with X and irX:


These are all corner cases, though. I guess using not X would work in most situations, at least for you to be understood.

Many valid points in the three other answers so far, feel free to unaccept mine and pick another one!

  • 2
    ... and sometimes they mean the same thing (flammable, inflammable)
    – abligh
    Commented Aug 28, 2021 at 15:49
  • @abligh That’s because inflammable came from the same Latin word as inflame, and the in- in inflame comes from a preposition like on in “on fire.” However, some people incorrectly thought the in- had the same effect as in “incorrectly”—or at least safety authorities were afraid they would and cause an accident. So, starting in the 1980s, warning labels were required to say “flammable,” and that really did confuse a lot of people.
    – Davislor
    Commented Aug 28, 2021 at 18:11
  • @abligh Doctors, I guess, figured that other doctors and nurses would know better than to think inflammable meant “no risk of flammation.”
    – Davislor
    Commented Aug 28, 2021 at 18:19
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    Note that irregardless is often considered incorrect in standard English
    – Tim
    Commented Aug 28, 2021 at 22:42
  • @tim Yes, this word is non standard and controversial indeed.
    – jlliagre
    Commented Aug 28, 2021 at 22:57

Also, regarding the negative sentences such as,

  • "It was not an unpleasant journey" vs.
  • "It was a pleasant journey"

The meanings are not equivalent. The first sentence means that there were no incidents, etc. to spoil the experience while the second emphasizes that the person making the journey actively enjoyed it.

This answer seems a duplicate of the accepted answer by @jillagre, but I aim to show that the nonequivalence may hold even when using adjectives which the OP gave examples of.


To add to James' answer on the word "happy", note that "really unhappy" is different from "not really happy", and although "not" in the second phrase modifies "really", one might be tempted to use that phrase if one tries to avoid the "un-" prefix.


The question was about adjectives that take un. The question is not about negative prefixes in general. However, I have given an overview below taken from a very good online site.

unpleasant = not pleasant
unbelievable = not believable [excluding the slang-ish meaning of unbelievable]
unchanged = not changed
untarnished = not tarnished
unhappy = not happy
unhinged = not hinged [as a door not being hinged, not the slang-ish meaning of unhinged] unfriendly = not friendly

However, if the word changes as in dishonest, you can also use: not honest

dishonest = not honest
disused = not used

Negative statements are the opposite of affirmative statements. In English, one way to make negative statements is by adding negative prefixes to nouns, adjectives, and verbs. Here are some English negative prefixes: a-, dis-, il-, im-, in-, ir-, non-, un-. For example, the prefix un- can be attached to the adjective happy to create the negative adjective unhappy. Or you can use the negative adverb not. Note that there is no difference in meaning between these two forms. Un- Prefix negative prefixes

More examples with other prefixes: abnormal = not normal
disassembled= not assembled
illiterate = not literate
immaterial = not material
insensitive = not sensitive
irreverent = not reverent
non-vaccinated = not vaccinated
unappealing = not appealing

Not all words that appear to have a negative prefix actually do: irrigate, for example. [Also, from that website].

If a word like honest has a negative prefix like dishonest, you can't just change the prefix but you can always use: not honest.

For the full story, please visit the link.

  • 1
    Not sure about some of the examples. In "disused" and "disassembled" (but not "dishonest"), "dis-" carries the meaning "not any more" rather than "isn't and maybe never was", which is what "un-" and "not" mean in both cases. I don't know if I've heard "non-vaccinated" rather than "unvaccinated". And technically, "ab-" isn't a negative prefix, but one meaning "from, away from"... Commented Aug 29, 2021 at 16:48
  • @TimPederick a disused factory, a factory that is not used. A disassembled bicycle, a bicycle that is not assembled. unvaccinated an non-vaccinated would not mean the same thing. And Reuters uses the term non-vaccinated: reuters.com/world/europe/…
    – Lambie
    Commented Aug 29, 2021 at 16:53
  • Also, I didn't say ab was a negative prefix. The text I quote says: a, not ab. The bike was lying there disassembled (or not assembled) on the garage floor.
    – Lambie
    Commented Aug 29, 2021 at 16:55
  • (1) They can indeed mean the same thing, but I'm pointing out a distinction between "dis-" and "not" that may be important. Per Oxford/Google: disused, no longer being used; disassemble, take something to pieces. It was used/assembled, now it's not. "Not" is broader, including this and the "never was" case, so it may not be "safe" (in the asker's terms). (2) I hadn't heard "non-vaccinated", so thanks for the citation. But why do you say "un/non-vaccinated" would not mean the same thing? (3) "Ab-" appears in your "abnormal". But pointing it out is just me being nitpicky. :-) Commented Aug 30, 2021 at 3:31
  • @TimPederick Where does it say that negative prefixes all have to mean the same thing? Oh I did say abnormal. Well, abnormal is not normal. So it is a negative prefix in that term, which I now remember is why I put it in. But let's make sure we upvote answers that don't provide an overview.
    – Lambie
    Commented Aug 30, 2021 at 13:11

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