Learn Hardcore Japanese: watasi ha gogo ni tosyokan de benkyousimasu.

は marks the topic: it sets up “as for me.” It doesn’t say who grammatically performs the action so much as what we’re talking about. In this sentence, 私 is the topic; the verb and other parts describe what’s true about that topic. If you used が instead (私が…), you’d be emphasizing “I (and not someone else)” as the doer, usually in an answer to “who?” or to contrast with others.

Do I need to say 私 at all?

Often no. Japanese usually omits subjects when obvious from context. 午後に図書館で勉強します is perfectly natural as “(I) study at the library in the afternoon.” Say 私 when you need to clarify or contrast (e.g., when several people are involved).

What does に after 午後 do? Is it required?

に marks the time when something happens. With broad time expressions like 午前/午後/朝/夜/週末, に is optional: both 午後に勉強します and 午後、勉強します are fine. With exact times/dates (3時、10月1日、月曜日) you generally use に (e.g., 3時に). With words like 今日・明日・昨日・毎日, に is usually not used.

Why is it 図書館で and not 図書館に?

で marks the location where an action happens (place of activity). に marks destination or the place of existence. So:

図書館で勉強します = I study at the library. (action location)
図書館に行きます = I go to the library. (destination)
図書館にいます = I am at the library. (existence)

Can I say 図書館に勉強します?

No, that’s unnatural. Use 図書館で勉強します. If you want to say “I will go to the library to study,” say 図書館に勉強しに行きます (literally, “go to the library in order to study”).

What tense is 勉強します? Does it mean present or future?

Japanese has a “non-past” form that covers both present and future. 勉強します can mean “I study (generally)” or “I will study,” depending on context. If you need “right now,” use the progressive 勉強しています.

What’s the difference between 勉強します and 勉強する?

Same meaning; different politeness. 勉強します is polite; 勉強する is plain/casual. Use polite forms with strangers, teachers, or in formal settings; plain with friends/family. Examples:

Polite past: 勉強しました
Polite negative: 勉強しません
Plain past: 勉強した
Plain negative: 勉強しない

There’s no object. Is that okay? How do I say what I study?

It’s fine; 勉強する can stand alone as “to study.” To specify the subject, add an object with を:

日本語を勉強します = I study Japanese. You can combine with place/time: 午後に図書館で日本語を勉強します.

Can I change the word order?

Yes, because particles show each part’s role. The default, most natural order is roughly: Topic > Time > Place > Object > Verb.

Default here: 私は午後に図書館で勉強します
Also OK: 私は図書館で午後に勉強します (slight emphasis on location first)
Using a comma to topicalize time: 午後、図書館で勉強します Keep the verb at the end; big reorders are possible but can sound marked or emphasize contrast.

Does 午後 mean “this afternoon” specifically?

By itself, 午後 is “afternoon” and can be understood as general or, from context, the upcoming afternoon. To be explicit about today, say 今日の午後 (optionally 今日の午後に).

What’s the difference between 午後に and 午後は?

午後に answers “when?” for an event: a neutral time marker.
午後は makes “afternoon(s)” the topic and often implies contrast or a habitual schedule: 午後は図書館で勉強します = “As for the afternoon(s), I study at the library” (maybe mornings are for something else). It can also mean “This afternoon, (as for that time,) I’ll study at the library,” with a contrastive feel.

Are spaces normal in Japanese writing?

No. Spaces are added in teaching materials to help learners. Normally it’s written without spaces: 私は午後に図書館で勉強します。

How do I pronounce は here?

When は is the topic particle, it’s pronounced “wa.” So the whole sentence reads:

Hiragana: わたしはごごにとしょかんでべんきょうします。
Romaji: Watashi wa gogo ni toshokan de benkyō shimasu.

Is 勉強をします different from 勉強します?

Both are correct. 勉強します is more common and a bit lighter. 勉強をします can sound a touch more formal or deliberate, but in everyday speech you’ll mostly hear 勉強します.

How would I say “I’m at the library (now)” versus “I’m studying at the library (now)”?

Existence/location now: 図書館にいます。 (“I am at the library.”)
Ongoing action now: 図書館で勉強しています。 (“I am studying at the library.”)