The one that happened first (had done) (A2 Grammar)

To point to a past action that happened before another past action, use had + the past participle. It is had for everyone (I, you, he, she, we, they), never has or have here.

had + the past participle

The same past participle, now after had

You already build the past participle for have / has sentences: most verbs just add -ed (work → worked, finish → finished, play → played), and some common verbs are irregular, so you learn each one. It is exactly the same form here. What is new is the word in front of it: had instead of have or has.

Saying no and asking (hadn't / Had…?)

To make it negative, put not after had: had not → short form hadn't, then the past participle. To ask, start with Had + the person + past participle? Short answers: Yes, … had. / No, … hadn't. Notice you never use did here — had does the work.

The earlier one gets had

When two things happened in the past, the earlier one takes had + past participle. The later one stays in the plain past.

Just one past action? Keep it plain

If you talk about only one past action, you do not need had. Use the plain past, like normal.

Putting it together: which verb gets had?

When a sentence has two past actions, decide the order: the earlier action takes had + past participle, and the later action stays in the plain past. Words like by the time, when, before, and after often signal the order.