Learn Hardcore Japanese: watasi ha zyugyougo ni tosyokan de benkyou simasu.

は marks 私 as the topic of the sentence (“as for me”). It implies you’re talking about what you do. If you used が (the subject marker), it would emphasize “I” as the subject in contrast to someone else, which isn’t the nuance you need here.

Can we omit 私 altogether?

Yes. Japanese often drops pronouns when context makes the subject clear. So you can simply say 授業後に図書館で勉強します and it still means “I study in the library after class.”

Why is に used after 授業後?

When you attach に to a noun of time (like 授業後 “after class”), it marks a specific point or moment when the action takes place. So 授業後に means “at the time after class.”

Could you leave out the に after 授業後?

In casual speech you might hear 授業後図書館で～, but formally you use に to avoid ambiguity, especially in writing.

What does で indicate in 図書館で?

The particle で marks the location where an action occurs. Here it tells you “the place of studying” is the library.

What’s the difference between で and に for locations?

Use で when you describe where an action happens (勉強する, 食べる, 話す). Use に when you indicate destination (行く, 来る) or existence (いる, ある).

Why is there no を before 勉強?

勉強する is a verbal noun + verb construction. You can optionally say 勉強をします, but it’s equally correct to say 勉強します without を. Omitting を is common and sounds more natural here.

Why does the verb come at the end?

Japanese follows a Subject–Object–Verb (SOV) order. All modifiers (time, place, objects) appear before the verb, which always finishes the clause.

Could you switch the word order around?

You have some flexibility, but time expressions typically come early (授業後に), location next (図書館で), and then the verb. If you swap them too freely, it might sound odd or change emphasis.

Why is します used instead of する?

します is the polite (masu) form of する. In casual speech or writing, you could say 勉強する. In a polite context—classmates, teachers, or emails—use 勉強します.