1

Which one is correct and why?

I can speak (a language).

I know to speak (a language).

3

I can speak English (a language).

I know to speak English (a language).

Both the sentences are grammatical, but there's a difference in meaning.

The former means that you have the ability to speak English.

We don't commonly use the structure "know + to-infinitive". We usually use the pattern "know how + to-infinitive". So if you say "I know how to speak English", it means that you have the necessary knowledge to speak English. Hence, this sentence is very close in meaning to the first sentence "I can speak English"

As for the second sentence "I know to speak English", it's not ungrammatical. It conveys the sense that you know that you should speak English.

  • +1, but "know + to-infinitive is quite common although never with the meaning "to know how". If my car starts making noises, I know to take it to a mechanic. The children know to line up and leave the building in an orderly manner during a fire drill. – Tᴚoɯɐuo Dec 25 '16 at 12:56
1

The first one is correct.
The second almost, it should be

I know how to speak (a language).

(at least when you want it to have the same meaning as the first sentence; 'I am able to speak (a language)').

A dictionary will tell you that can and know how to are, in some circumstances, synonymous. Especially when it comes to complex skills.

As you indicated in your comment, for languages, the following form can be used as well:

I know (a language)

  • What about I know (a language). ? – Ľubomír Masarovič Dec 24 '16 at 11:27
  • Yes, that would work as well, but it is mostly used for languages and not for other skills. – Glorfindel Dec 24 '16 at 11:30
  • @Lubomir Masarovic "I know (certain language)" is correct but it doesn't mean that you can also speak that language. Just for additional information.. – Hassan Ashas Dec 24 '16 at 11:47
  • @HassanAshas 'I know Spanish' usually means 'I am able to speak (some) Spanish'. For some languages, e.g. Latin and Old Greek, it probably doesn't. – Glorfindel Dec 24 '16 at 11:54
  • @Glorfindel Oh, I didn't know that, lol.. Tho, grammatically, "I know English ( or whatever )" wouldn't mean that I am also able to speak English.. That's what I was trying to say ^_^ – Hassan Ashas Dec 24 '16 at 11:56

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