Long 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”.
 Tip: Are you a non-native English speaker? I have just finished creating a
 Tip: Are you a non-native English speaker? I have just finished creating a  Web App
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