Maintained is already the past participle of maintain. It's also the past tense.
No, "maintained" is not a preposition. It is a past tense verb that means to keep something in a particular condition or to keep it in existence.
The past participle of am is been. Not does not have a past participle
The past participle of "do" is "done." The past participle of "have" is "had."
The past tense of "have" is "had," and the past participle is also "had."
The past and past participle for "buy" is "bought."
Being is the present participle. The past participle is been.
The past participle is thought.
Eating is the present participle; eaten is the past participle.
"Did" is the past and "done" is the past participle.
The past participle is had.
The past participle of "do" is "done."
The past participle is done. The simple past tense is did. The present participle is doing.