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How does using "not" differ from using a word with a negative prefix?

For example:

It's unwise to vote down this question.
It's not wise to vote down this question

I know semantically (or literally), they're the same. However, I want to know grammatically, how do they differ?

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  • 1
    By "technically", do you mean that you want to know the grammatical terminology to describe the difference?
    – Ben Kovitz
    Jun 4 '15 at 17:12
  • Yes. according to the facts or exact meaning of something; strictly (from Google, 1st definition)
    – XPMai
    Jun 4 '15 at 17:23
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    "Define technically" does not specifically mean "supply the grammatical terminology". "Technical" is actually a vague word in English, despite the fact that it suggests a kind of preciseness. Contrasting "semantically" with "technically" doesn't really make sense. A reader wonders, "What kind of preciseness do you have in mind?" BTW, referring someone to lmgtfy.com is a little bit insulting, as it suggests that I was careless and could easily have looked up the definition for myself. But if you look at the results, the definition doesn't mention grammatical terminology.
    – Ben Kovitz
    Jun 4 '15 at 17:33
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    "Specifically" is not any better than "technically". You want "grammatically"
    – Catija
    Jun 4 '15 at 17:49
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    Actually, in my first comment, I asked you for clarification. It was not a criticism. The clarification is needed to write a helpful answer.
    – Ben Kovitz
    Jun 4 '15 at 18:26
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Grammatically, unwise and not wise are nearly identical. Unwise is an adjective, while not wise is an adjective phrase.

To expand on the subject in general, you can add the prefix un- to many adjectives to form a negative form. Other examples are unclear, unaware, and unpleasant. Some words take a different prefix, such as il- in illegitimate (the opposite of legitimate), or dis- in dishonored, in which case the un- form does not exist or is very uncommon. Unfortunately most of these must be learned by rote (you have to memorize it for each word), as there isn't a good way to know without looking it up.

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  • Can you simplify this part, "To expand on the subject in general,", I don't understand what it means.
    – XPMai
    Jun 5 '15 at 14:34
  • @XPMai I tried to clarify a little bit. What do you not understand specifically? Jun 5 '15 at 22:50
  • I don't understand the connotation. How to expand a subject? What subject you referring?
    – XPMai
    Jun 6 '15 at 1:06
  • "To expand on a subject" means to explain more about a particular idea. The subject, or idea, that I am referring to is negative adjectives. Jun 6 '15 at 15:26
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  1. It's unwise to vote down this question.
  2. It's not wise to vote down this question

Adding a prefix like un- to an adjective is grammatically very different from using the adverb not to negate a verb phrase. The most important difference is that using not makes the whole clause negative, whereas a clause containing an adjective with un- will be positive unless there is some other word that makes it negative.

One way that we can show this is to put a negative question tag onto the end of the clause. Usually, when the we have a positive clause we find a negative question tag. If we have a negative clause, we see a positive tag:

  • It's unwise to vote down this question, isn't it?
  • It's not wise to vote down this question, is it?

If we use the opposite tags the sentences will either be marked (have an unusual meaning), or be ungrammatical:

  • It's unwise to vote down this question, is it? (unusual/marked tag)
  • It's not wise to vote down this question, isn't it? (ungrammatical)

Another effect this has on the grammar is that there are words and phrases which we can use in negative clauses that won't work so well in positive ones. These are usually called negative polarity items. So there are words and phrases we can use with sentence (2) that we can't use with sentence (1). Take for example the negative polarity items either or at all:

  • *It's unwise to vote down this question either. (ungrammatical)
  • It's not wise to vote down this question either.
  • *It's unwise at all to vote down this question. (ungrammatical)
  • It's not wise at all to vote down this question.

So although the meaning of sentences (1) and (2) is very similar, the grammar is completely different!

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  • I can feel those 2 are wrong, but if I were asked to explain why, I can't. Can you? | Tenses are correct; sentence structure is correct; punctuation are placed at the right positions. What else that makes the 2 sentences wrong?
    – XPMai
    Jun 7 '15 at 10:55

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