Should I use the past tense with did? For example, I was to say:
The important question is: Did they knew what it means or not?
Or should I say:
The important question is: Did they know what it means or not?
In other words: Should I use past tense with did?
I looked at this:
Explanation of "did was [verb]" structure
but it didn't really answer my question.
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.
"Should I use the past tense with did" Answer: For the sentences you posted:
Did they knew what it means or not?
Did they know what it means or not?
The answer is this: any interrogative in English (except for the verb to be) is formed like this: Helping verb + subject + main verb.
The simple present tense, declarative: They know what this means
The simple present tense, interrogative: Do they know [etc.]
The simple past tense, declarative: They knew what this means
The simple past tense, interrogative: Did they know [etc.]
Therefore, /Did they knew/ is not grammatical.
For irregular verbs (except for be), the simple past interrogative form uses DID + the subject + MAIN VERB.
DO and DID are called the helping or auxiliary verbs used in Interrogatives.
The other question: /All I did was x/ is very different from what I explained above.
The DID in /all I did was/ is the past of the the main verb to do and is not a helping or auxiliary verb even though it is written the same way, the function of it is not the same:
Examples: All I do is teach English (present tense). Declarative.
All I did was teach English. (past tense of to do). Declarative
