The third form of the present tense in English is for emphasis. When you use do as an auxiliary in an affirmative sentence, it makes the verb stronger:

–Do you speak English or French?

–I don't speak French, but I do speak English.

–He does understand you, doesn't he?*

*Do for emphasis is also used in a sentence with a tag question when you want to be sure you have the right information.

Examples of Emphatic Present

• I don't speak Russian, but I do speak English.
• He doesn't like apples, but he does like oranges.