4) Interrogative sentences:

To form the interrogative past form of the auxiliary verbs Be or Can, you just need to put the forms "Was", "were" or "could" before the subject

-They were students ==> Were they students?

- John could swim when he was 5==> Could John swim when he was 5?

To form the interrogative form of any other verb in the Simple Past you need to use the past form of the auxiliary verb do - that is did + the subject + the infinitive of the verb you want to use, no matter if that verb is regular or irregular:

- Peter bought a new car==> Did Peter buy a new car?

- Sam waited for her==> Did Sam wait for her?