Learn Hardcore Japanese: kanozyo ha hudan tosyokan de benkyousuru ga, sensyuu ha ie de benkyousita. - She usually studies at the library, but last week she studied at home.

彼女かのじょ　は　普段ふだん　図書館としょかん　で　勉強べんきょうする　が、　先週せんしゅう　は　家いえ　で　勉強べんきょうした。kanojo wa fudan toshokan de benkyousuru ga, senshuu wa ie de benkyoushita.

She usually studies at the library, but last week she studied at home.

Breakdown of kanojo wa fudan toshokan de benkyousuru ga, senshuu wa ie de benkyoushita.

Questions & Answers about kanojo wa fudan toshokan de benkyousuru ga, senshuu wa ie de benkyoushita.

Why is the first verb non-past (勉強する) while the second is past (勉強した)?

Japanese uses the non-past to state general, habitual facts. So 普段…勉強する means she usually studies. The second clause reports a specific event in the past, so 先週は…勉強した is past. Mixing non-past (habit) and past (one-time event) is normal.

What does は do after 彼女 and after 先週?

彼女は sets the topic: we’re talking about her.
先週は is a contrastive topic: as for last week (contrasting it with the usual pattern).
Two different uses of は can appear in one sentence.

Could I use 彼女が instead of 彼女は?

You could, but it changes the nuance. 彼女が focuses on her as the doer (she, as opposed to someone else), not as the topic of discussion. Here, 彼女は (topic) is more natural. 彼女が普段図書館で勉強する often works better inside a larger sentence or when emphasizing “she (not others) is the one who usually studies at the library.”

What kind of が is the one between the clauses? Is it the subject marker?

Here が is a conjunction meaning “but/although,” not the subject marker. You can tell because it connects two clauses: …勉強するが、先週は…. The subject 彼女 is already marked by は at the start.