Interrogative sentences in the simple present and simple past tense are formed by putting do, does or did before the subject.
Do is used in the simple present tense when the subject is I, you or a plural noun or pronoun. Does is used in the simple present tense when the subject is a singular noun or pronoun.

Did is used in the simple past tense with subjects of all number and person.

• She came. (Assertive)
• Did she come? (Interrogative)
• I wrote a letter. (Assertive)
• Did I write a letter. (Interrogative)
• I know the answer. (Assertive)
• Do I know the answer? (Interrogative)

Related posts:

1. Formation of interrogative sentences
2. The Sentence
3. Changing statements into questions
4. How to change a statement into a question