The form
will have + past participle — and will have does not change with the person.
| Subject | will have | Past participle |
|---|---|---|
| I, you, we, they | will have | finished |
| he, she, it | will have | finished |
I will have finished. · She will have left. · They will have started.
No has. No had. That is the whole simplification.
It shrinks in speech
- I will have → I'll have
- She will have → She'll have
- They will have → They'll have
Keep the have. I'll finished is missing the helping verb that the participle needs.
The participle is the same one
Written, gone, done, seen, trained — the third form does not change just because the helping verbs did. If that list still wobbles, that is The Past Participle, with Maya and Theo.