The form
had + past participle — and had does not change.
| Subject | had | Past participle |
|---|---|---|
| I, you, we, they | had | finished |
| he, she, it | had | finished |
I had finished. · She had left. · They had started.
No has, no have. That is the whole simplification from the present perfect.
It shrinks in speech
- I had → I'd
- She had → She'd
- They had → They'd
I'd can also be I would. The word after it tells you which: I'd go (would) versus I'd gone (had).
The participle is the same one
Written, gone, done, seen, trained — the third form does not change just because the helping verb did. If the participle is the part that still wobbles, that is The Past Participle, with Maya and Theo.