did + present tense (or ) Past tense of the verb

What did you say?
What you said.

How did you and mum guess I wanted Agnes?
how you and Mum guessed I wanted Agnes

What is the meaning of above two sentences?
When should I use “did + present tense” and the past tense of a verb?

Answer

"Did" is often used in English in questions to turn a statement into a question.

People who are learning English are often tempted to turn a statement into a question by simply adding an interrogative word like "how" or "why" to it.

Statement: You gave him the box.

Attempt at question – WRONG: Why you gave him the box?

To make a proper question, you must change the verb from past tense to "did" plus present.

Question – RIGHT: Why did you give him the box?

I can’t give you a good reason why you need to do this. It’s just how it’s done in English.

Perhaps I should clarify that that’s not the only way to phrase a question.

If the action is in the future, we are already using a helping verb like "will". In this case we move the word "will" to precede the subject, but otherwise keep the same verbs.

Statement: You will give him the box.

Question: When will you give him the box?

Likewise if the original statement is in the present continuous, like "are giving".

Statement: You are giving him the box.

Question: Why are you giving him the box?

If the statement is in the present tense, we add the word "do".

Statement: You think the box is full.

Question: Why do you think the box is full?

Attribution
Source : Link , Question Author : Amish Aa , Answer Author : Jay