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I was reading a website where common English sentence errors are written -
INCORRECT - He doesn't know to swim.
CORRECT - He doesn't know how to swim.
I know He doesn't know how to swim is a correct sentence. But why is the other one wrong, is my question?
I know that know is a transitive verb. So, the following are correct -
i) He knows swimming.
Swimming is a gerund. We can also use the infinitive form here - to swim
ii) He knows to swim.
Whether to include how or not isn't a matter of of correct/incorrect. They mean different things...
1: "He knew to stay out of sight"
...means he knew that it was necessary/desirable for him to remain unseen.2: "He knew how to stay out of sight"
...means he knew one or more methods by which he could remain unseen.
In most contexts where sense #1 applies it's strongly implied that the person who knows that an action is required either has done it or will definitely do it (and thus by further implication, knows how to do it).
Taking note of comments below, I think it's worth pointing out that although to know how to do X usually implies to be able to do X, this isn't inevitably the case...
A consultant is a eunuch: he wants to, he knows how to, but he can't actually do it.
(It's quite possible to know how to do something in theory, but be unable to do it in practice.)
The problem with the first sentence is as follows and this is an issue of CONTEXT
"He doesn't know to swim" would imply that he should be swimming but doesn't know it. i.e. Jim entered the triathlon and should be swimming but doesn't know it.
The correct way would be:
"He doesn't know how to swim" – Jim entered the triathlon but doesn't know how to swim
The other answers told you the meanings were different, but I feel like they're missing something.
The reason "He knows to swim" is incorrect for the meaning you intend is that "know" has multiple meanings in English, so you have to disambiguate it by changing the rest of the sentence.
If you want "I know X" to mean "I am aware that I am supposed to do X", then you probably shouldn't put "how" after it. (e.g. "My dog knows to come when called.")
On the other hand, if you want "I know X" to mean "I have learned X" (X is a action or practice here) then you should probably put "how" after it.
The error described by the OP is a common example of false friend. A number of languages use the same word to express two different usages:
I know who Alice is. She's Bob's sister.
I can swim. I learned to swim when I was five.
Thus, the short answer is that one needs to learn that in English this usage is expressed using the verb "can".
I won't attempt to describe the use of know + to-infinitive as FumbleFingers already does a fine job in his answer:
He knew to stay out of sight
I will, however, list the few usages of know + PP-headed-by-to that I have come across:
He was known as Bonzo to his friends.
As one of the founding teachers of the high school, Mrs. Stupin, as she was known to her students at the time, was caring, creative, thought-provoking, entertaining and enthusiastic.
Much of what he has written there was known to me and it has found its place in my reminiscences as a Theology student.
Everything You Ever Need to Know to Hypnotize Yourself
The Words You Should Know to Sound Smart
PP stands for prepositional phrase, i.e. a phrase headed by a preposition.
The correct and direct answer to this question is in snailboat's comment:
"He doesn't know to swim" is a grammatical and meaningful sentence, but it does not mean the same thing and it is significantly less common
As stated in fumble_fingers's answer, "He doesn't know to swim" is equivalent in meaning to "He doesn't know he should swim." Yes, it is a grammatically valid sentence.
However, it is not common. The [he/she knows]+[infinitive] construction is fairly rare and I suspect it might be more common in BrE than AmE.[1] This is compounded by the use in the negative ("He doesn't know to swim"), which would be even less likely to come up -- maybe if someone is on the deck of a sinking ship, but she doesn't know it is sinking? I would much more frequently say "She doesn't know she should swim" to achieve that meaning.
So I would also hazard a guess that the creator of the website that OP references is perhaps also an AmE speaker, and it didn't occur to him or her that "he doesn't know to swim" is a "correct" sentence in some circumstances because it is so rare.
Also, the underlying meaning "Do you have the ability to swim" also comes up much more frequently (dozens of times in my life, perhaps more), such that "know how to swim" is almost an idiomatic construction due to its frequency of use.
By contrast, a context for the meaning "Do you know it is appropriate and needful for you to swim now?" comes up . . . almost never, only on sinking ships and similar swimming-needful crises. So the language learning site was just trying to illustrate the most common scenario.
If you're an ELL student, it's much more likely that you want to know if your friend can swim, so you can invite him to a pool party, rather than whether he should swim.
[1] Fumblefingers is in the UK. Both FF's linked book examples are US authors, so I guess that doesn't help me, but we do have me, snailboat, and GalacticCowboy (all AmE speakers) saying it seems like an unusual or uncommon construction.
The people talking about context are correct:
Basically there are 6 fundamental questions Who? where? what? Why? Where? and how? In a natural language these are all answered in a complete sentence, or can be inferred from the context. To do this the sentence needs an object (it already has a subject and verb).
He knows to (where, how, when, why...) swim.
Without context, I have to assume too much about the point the writer is making, and what to associate with the word 'knows'.
Therefore the sentence is meaningless. Even if I supply context, this sentence can be interpreted in many different ways depending on how I fill in the missing context. Different readers are highly likely to come up with vastly different interpretations.
Humorists and writers often use this device in the 'twist of the tale' story where you are intentionally led to an incorrect scenario. This has to be done with some subtlety or the reader 'loses the plot'. This is achieved through building context.
Even with context, the sentence without an object may be too open to interpretation. See the interpretations above; also consider:
The fugitive came to the river, he could hear the chasing dogs and police in the distance getting nearer. He knows to swim. (What does he know about swim?)
Other people have addressed the first part of your question by pointing out that "knows to swim" does mean something, and it means something different than "knows how to swim". I just want to clarify that the second part of your question is also bringing up an incorrect equivalence:
I know that know is a transitive verb. So, the following are correct -
i) He knows swimming.
Swimming is a gerund. We can also use the infinitive form here - to swim
ii) He knows to swim.
While we might say "he knows swimming", it is because the word swimming has been transformed by usage into a noun meaning "the sport of swimming". You could be a journalist that "knows swimming" even if you don't know how to swim.
Other gerunds would sound weird there--e.g. "he knows thinking" or "he knows exercising"--because those activities have not become an event/profession/sport/whatever. "He knows to think" and "he knows to exercise", again, would be "he knows that he should think [in this situation]" and something like "he knows that he should be exercising [regularly]". And others just really wouldn't work at all unless you are deliberately "nouning" it. "That was annoying, and I know annoying." (said as a joke where people know that that person experiences annoyances a lot or causes annoyances a lot).
But "he knows existing"? "he knows to exist"? These don't really have any meaning.
Know how to means “have the ability to”. Know means “have knowledge of”.
If I said I know to swim, it might mean “I know I should swim”, but can't mean “I have the ability to swim”. Statements regarding ability need how.
I have come across some instances wherein 'know to [verb]' has been used instead of 'know how to [verb]'. And, I learned from an entry on Wiktionary that this is an archaic use.
In some old texts, the form "know to [verb]" rather than "know how to [verb]" is found.
An example from the Google Books results:
"He knew himself to sing, and build the lofty rhymes." - School Virgil by R.J. Thornton
The book was written in the early 18th century.
Maybe in Modern English the sentence is not widely accepted (I had asked a question about whether the sentence "He is come" was correct. It's correct but then today, we have better structure to say that!). That said, it's correct but the rules have changed and so has the style of writing/speaking.
The syntax of "know...to" (not "how to...: or "know that...") must be a special use or idiomatic use of the verb "know" in British English, no doubt about that. The British (especially Scottish, even Irish) idiomatic stimulus is full of uses not really grammatically or syntactically correct, but familiar to the listener, especially in everyday speech / spoken English, thus acceptable.
However, sometimes we encounter the verb "know" as a transitive verb, used mostly with perfect tenses, with the sense of knowing something from past experience, as in the sentences:
I haven't known this to happen in all these years I make egg and lemon sauce;
I've never known him to be aggressive (=he's never aggressive);
but
I know him to be a great liar (=I've exprerienced him being a liar in the past, he's a known liar);
