A plain-form clause placed before a noun describes it: ［…が 作った］ケーキ.
これは ミラーさんが つくった ケーキです。
This is the cake that Mr Miller made.
Japanese has no relative pronoun (“that/which”). The describing clause simply goes in front of the noun, with its verb in plain form. Inside the clause the subject is marked with が (not は).
The modified noun can play any role, e.g. the topic of the main sentence.
きのう みた えいがは おもしろかったです。
The film I saw yesterday was interesting.
The clause + noun unit behaves like any noun phrase and can take は, が, を, etc. Here ［きのう 見た 映画］は is the topic of the sentence.
もう + past (〜ました) = “already done”. “Not yet” is まだ + 〜て いません — NOT the plain past negative.
I have already done the homework.
No, I haven't done the homework yet.
〜ました covers both “did (yesterday)” and “have already done”. With もう (already) it means the action is complete.
For “not yet”, don't use ませんでした (that is a finished past, “didn't”). Use まだ … て いません — the action still hasn't happened.