sentence structure - What did you say / said?

What is the correct sentence :

1. What did you say?

or

2. What did you said?

The second sentence seems correct, but I am not certain.

Answer

The correct version is

What did you say?

The simple reason for this is that when you form a question like this in English, you use an auxiliary verb, in this case "do". When the question is in the past tense, you conjugate the auxiliary verb, but the main verb is just the bare infinitive, "say", and never changes.

What does he say?
What did he say?
What will he say?
What could he say?

Notice that "say" never changes, no matter how the auxiliary verb changes.