The teacher has a busy timetable and has forgotten that you have already looked at this topic in a previous lesson. You may say:
"We've already covered this," or, "We've already done this," meaning that you worked through this subject matter before.
"Teacher, sorry, I think we've already covered this."
"Oh, very sorry. Well let's skip to the next unit."
先生は時間割が詰まっているので、前回のレッスンでこのトピックをやったことを忘れたのでしょう。
以下のように言うといいでしょう。
"We've already covered this"
または
"We've already done this"
（もうこれはやりました。）
こちらの表現は、以前このトピックをやったという意味です。
会話例
"Teacher, sorry, I think we've already covered this."
（先生、すみません、もうこれはやったと思います。）
"Oh, very sorry. Well let's skip to the next unit."
（あ、申し訳ないです。次のユニットまで飛ばしましょう。）
We have covered that already. Can we do Discussion Materials instead.
*We have covered that already. Can we do Discussion Materials instead.
This means that you do not want to do the material that the teacher was suggesting because you have done it before but you would to do something else.
To cover something means to have done it before.
*We have covered that already.
Can we do Discussion Materials instead.
これはあなたが以前その先生が提案してきた教材を使ったので使いたくなく、他のことをしたいということを意味しています。
To cover somethingはすでに以前終わっているということを意味します。
"We've already been through this section." This is a way of telling your teacher that what you are going over has already been covered (been over/through).
"We haven't looked at this section yet." This is a way of telling the teacher that you haven't read or looked at something yet. And is a polite way of drawing (directing/guiding/showing) their attention to it.
"We've already been through this section."（この部分はもうやりました）
- これは、その部分はもうやったことがあると先生に伝える言い方です。
"We haven't looked at this section yet."（この部分はまだやっていません）
- これは、その部分をまだやっていないことを伝える丁寧な言い方です。
"to go over (something)" - this means to go through the material, to complete to the material
A. I don't want to do this lesson, teacher.
B. Why not? It's very useful to know.
A. I've already gone over this in another class, but I haven't done the other lesson yet.
"to go over (something)"（～はしました）
- これはこの教材はやりました、終わりました、という意味になります。
【例文】
A. I don't want to do this lesson, teacher.
（このレッスンはやりたくないです、先生）
B. Why not? It's very useful to know.
（なぜですか？知ってるととても便利ですよ）
A. I've already gone over this in another class, but I haven't done the other lesson yet.
（他のクラスでこれはすでにやりました、でも他の物はまだやっていません）
While you go through a lesson you are "using" the material to learn so you can say "We've used this material before."
"We haven't used this material yet." You can say "haven't used yet" which means that until now this material hasn't been used.
"We completed this exercise already." You can say "complete this exercise" to indicate that you finished or completed a specific exercise already.
レッスンをしているときは、教材を "using"（使っている）しています。
ですから、
"We've used this material before."（この教材は使ったことがあります）
と言えます。
"We haven't used this material yet."（この教材は使ったことがありません）
"haven't used yet" は、その教材をまだ使ったことがないことを表します。
"We completed this exercise already."（このエクササイズはもう終わりました）
"complete this exercise" はそのエクササイズを既に終えていることを表します。
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