Tip: See my list of the 
MostLong story short, if you want to say that you can express yourself in English and/or that you can communicate in English, the natural way to say this is “I speak English”, as in the following example:
Yes, I speak English well.
When you say “I know English”, it sounds like you are treating English as a theoretical subject. That is not something people normally say about a living language like English, but it would make sense for a classical scholar to say:
because Ancient Greek is a language people learn for the sake of having knowledge about the language.
When you refer specifically to a rudimentary knowledge of English, it is possible to use “know”:
No, he knows very little English.
Nevertheless, if you speak about any other situation, such as when someone understands a language but doesn’t speak it, it is better to describe the situation in full, i.e. “he understands English but isn’t able to speak it”, instead of using the verb “know”.
