Past-Tense – Should the Past Tense be Used After ‘Before’?

Generally speaking, only the first verb in each clause is finite:

They knew what it meant.

Here, know is the finite verb. It changes form to agree with the subject (I know, she knows) and to indicate tense (They know, they knew).

To turn this into a question, we need to apply Subject-Auxiliary Inversion. But to do that, we need an auxiliary, so we add the dummy auxiliary do:

They did know what it meant.

Did they know what it meant?　　←　did and they are inverted

Now do is the finite verb. It changes form to agree with the subject (I do, she does) and to indicate tense (They do, they did). The second verb, know, is non-finite and does not change form.

*Did they knew has two finite verbs and is ungrammatical.

We use the infinitive (not the present tense) after did (the axillary verb) in questions and negative sentences. This is correct.

Did he eat his lunch, and go to his father’s shop this morning?

We usually use the past form of the verb in the affirmative sentence. This would be correct.

He went to his grandma’s home yesterday, but his sister went to her school.

But you can use emphatic did with the infinitive in the affirmative sentence in some situations to emphasize the action.

He did go to his grandma as he had promised, but his sister didn't. She went to her school.