That would be "reached".
The past is reached.
The past tense of "reach" is "reached."
The past participle of "reach" is "reached."
The past tense of "reach" is "reached" and the past participle is also "reached."
The past perfect tense of reach is had reached.
Reach the other is past tense
No, it is not a noun. Reached is the past tense and past participle of "to reach." The noun form is just "reach."
Decided and reached are past tense.
Reached. It sounds weird most of the time but fine when you don't think about it. ie "He reached for a glass of water"
No, it is not an adverb. Reached is a verb, the past tense and past participle of "to reach." There is no regular adverb form.
The former are yet to reach their peak, the latter are past it.
Doubtful.