もっとも (mind you)
though, mind you, having said that
Statement。もっとも、reservation。
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At the start of a sentence, もっとも adds a reservation to what was just said: 明日は休みだ。もっとも、午前中は少し仕事をするけど, "I'm off tomorrow. Mind you, I'll do a bit of work in the morning".
It softens or qualifies the first statement without overturning it. The second half often ends with けど, が or けどね.
It's close to ただし (N3), but ただし is a formal condition (on notices, rules), while もっとも is a personal afterthought.
As a な-adjective, もっともだ means "reasonable, natural": 彼が怒るのももっともだ, "it's only natural he's angry".
Examples
明日は休みだ。もっとも、午前中は少し仕事をするけど。
I'm off tomorrow. Mind you, I'll do a bit of work in the morning.
この店は料理がおいしい。もっとも、値段は少し高い。
The food here is good. It's a bit pricey, mind you.
彼の意見は正しい。もっとも、実行するのは難しいが。
His opinion is right. Though putting it into practice would be hard.
試験には合格した。もっとも、ぎりぎりだったけど。
I passed the exam. Only just, mind you.
毎日運動している。もっとも、散歩だけだけどね。
I exercise every day. Well, it's only walking.