1. Do
2. Does
3. Did
These helping verbs are used with the base form of the main verb to emphasize an action or statement.
Examples:
* Do you understand? (Emphasizes the question)
* He does like her. (Emphasizes the liking)
* I did finish my homework. (Emphasizes the completion)
Note:
* Do is used with the plural subjects and the pronoun *I*.
* Does is used with singular subjects (except *I*).
* Did is used with all subjects in the past tense.