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This question already has an answer here:

When using "did" in a sentence, does that put the verb in present or past tense? Example:

What did they teach you?

or:

What did they taught you?

Example 2:

What did he say?

or:

What did he said?

marked as duplicate by user24743, JMB, Nathan Tuggy, Glorfindel, StoneyB Feb 9 '16 at 11:13

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  1. What did they teach you?
  2. What did he say?

are the correct forms. Here the verb do is used as an auxiliary verb and it must be followed by a verb at the base form. So it's only do that undergoes the inflections according to the sentence form (interrogative, negative, affirmative) and it must be changed accordingly while the main verb is unaltered. Also have a look at do-support.

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