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.

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.

How does が compare with けど／けれど(も) and でも／しかし?

が: written/neutral-to-formal connector inside a sentence.
けど／けれど／けれども: spoken and flexible; けど is casual, けれども is more formal.
でも: starts a new sentence.
しかし: formal sentence-initial “however.”
All can express contrast, but register and positioning differ. Polite version here: 勉強しますが….

Do I need は after 普段? Should it be 普段は?

Both are fine:

普段 (adverb) simply means “usually.”
普段は adds topical/contrastive emphasis, often used when you’ll say what happens “this time instead.”
So 普段は図書館で勉強するが… highlights the contrast even more.

What’s the difference among 普段, いつも, たいてい, and よく?

いつも: always, every time (strongest).
普段: as a rule/typically (habitual baseline).
たいてい: generally/for the most part (broad tendency).
よく: often/frequently (frequency, not “usualness”).
Choose based on strength and nuance.

Why is で used after 図書館 and 家?

で marks the place where an action happens.

図書館で勉強する = study at the library
家で勉強した = studied at home
Use に for destinations (図書館に行く) or existence/location (家にいる).

Can I say 図書館に勉強する?

No. 勉強する takes the place-of-action with で, so it should be 図書館で勉強する. に would be used with verbs like 行く or 来る.

Why does 先週 take は and not に?

Relative time words like 先週, 昨日, 明日 usually don’t need に. You can say 先週勉強した. に is common with specific points in time: 6時に, 月曜日に, 2024年に. Here 先週は uses は for contrast.

Can I drop 彼女は?

Yes. Japanese often omits known topics: 普段(は)図書館で勉強するが、先週は家で勉強した。 Context will supply who “she” is.

What’s the nuance difference between 家, うち, and 自宅?

家（いえ）: house/home (neutral/generic; also for someone else’s home).
うち: my/our place, more homely; often used for one’s own home.
自宅: formal “one’s own home.”
In this sentence, 家で is fine for “at home.” If it were the speaker’s own home, うちで is very natural.

How would I say this politely?

彼女は普段図書館で勉強しますが、先週は家で勉強しました。
Keep the style consistent: both clauses in ます-form.

Can I change the word order of time and place?

Preferred order is broad-to-narrow: topic/time → place → verb.

Natural: 彼女は先週は家で勉強した (usually you’d keep only one は: 彼女は先週家で勉強した).
家で先週勉強した is possible but sounds less natural; time generally comes before place.

What’s the difference between 家で and 家では?

家で simply marks location. 家では adds contrast/emphasis on the location (“at home, as opposed to elsewhere”). In this sentence, 家では勉強した strongly contrasts with 図書館で and may feel redundant unless you want extra emphasis.

Could I use 勉強していた instead of 勉強した?

勉強した: studied (completed event).
勉強していた: was studying (ongoing state in that time frame), or describes a past habit.
先週は家で勉強していた suggests she was in a studying state throughout last week, not just that she did it once.

Why is there no object with 勉強する? Should it be を something?

勉強する can be used without an explicit object to mean “to study.” If you specify the subject, add を: 日本語を勉強する, 法律を勉強した. Your sentence is generic: “studies (something).”

Is the comma before が required? And what about the spaces?

The comma 、 is optional but helps readability. Standard Japanese writing does not insert spaces between words; the spaced version you saw is for teaching/clarity.