1A particle that marks the topic of a sentence (written は but pronounced 'wa'). It sets the theme or what the sentence is about.
私は学生です。
I am a student.
今日は雨です。
As for today, it's rainy.
この本はとても面白い。
This book is very interesting.
2A particle used to indicate contrast with another option (stated or unstated), often translated as '… (as for) X, (but) Y...'.
ビールは飲めるけど、日本酒は苦手です。
I can drink beer, but I'm not good with sake.
彼は行くと言ったが、私は行かない。
He said he'd go, but I won't.
昼は忙しいが、夜は暇です。
I'm busy during the day, but free at night.
3A particle used to add emphasis to a phrase—often used to stress contrast, negation, or emotion (e.g. 'そんなことはない').
そんなことはない。
That's not true (emphatic).
暑いは暑いが、海に行きたい。
It's hot, but I still want to go to the beach.
彼が悪いは悪いが、全部彼のせいではない。
He's at fault, yes, but it's not entirely his fault.