do support - Which to use "do" or "does" to emphasize with third person singular?

For example, I can say "I do like apples". May I similarly say

A kitten do like to play with a clew

or should I use does instead. Also, in sentences like

Only then do/does it really matter, which choice you make.

Answer

I (1st person singular) do like words.

A kitten (3rd person singular) does like words.

If you want to stress the verb in the second sentence you'll probably want to use an adverb. "A kitten truly does like words." The word do in your first sentence is part of the verb, not a modifier. I can see how you might be confused if you are learning English the way it is generally taught nowadays, i.e. without a formal study of English grammar.