"Let" is one of the "invariable" irregular verbs; its present, past, and past participle are all "let".
The past participle is also "let".
Let does not change from present tense to past tense. Let is also the past participle.
The past participle is had.
Done is the past participle.
The past participle is let. The simple past tense is letted.
The past participle of the word "Find" is the word "Found."
The past participle of the word "pick" is "picked".
The past participle of the word "finish" is "finished."
The word "invited" is the past participle of invite.
The past participle of "swear" is "sworn."
The past participle of the word 'fall' is 'fallen'.
The simple past and past participle are both welcomed.