Learn Hardcore Japanese: kyou ha youzi ga aru node, hayaku kaerimasu. - I have something to do today, so I will go home early.

は marks the topic. 今日 (today) is being set as the time frame we’re talking about: “As for today, ….” Time expressions commonly take は to topicalize them. It’s the most natural choice here.

Can I drop は after 今日 or use another particle?

Yes, you can say 今日、用事があるので… (dropping the particle) in casual speech. Using 今日が here is generally unnatural; が on time words is rare unless you’re strongly contrasting (“今日がダメなら…”). So use は or nothing.

Why is が used with 用事? Could I use を?

ある (to exist/there is) takes が for the thing that exists, so 用事がある is the standard pattern. 用事をある is ungrammatical. You could use は instead (用事はある) to contrast or highlight “as for errands, I do have them.”

What exactly does 用事 mean? How is it different from similar words?

用事 is a catch‑all for “something I have to do/attend to,” often personal and nonspecific. Contrasts:

予定: a scheduled plan/appointment on your calendar.
約束: an appointment/promise with someone.
仕事: work/job tasks.
事情/都合: circumstances/convenience (often used vaguely to avoid details). More formal than 用事: 所用 (shoyō) “some business/errand.”

Why ある and not いる?

いる is for living things; ある is for inanimate/abstract things. 用事 is an abstract “thing to do,” so 用事がある is correct. 用事がいる is wrong.

Why is it ある before ので and not あります? Is ありますので wrong?

The safe default is plain forms before ので: あるので. However, in polite/formal announcements and business writing, ありますので is also very common and acceptable. For nouns/na‑adjectives, you’ll see なので or ですので.

What’s the difference between ので and から here?

Both mean “because.” ので sounds more objective/softer and fits well with polite speech when giving reasons that affect others. から is more direct and can feel firmer or more personal. Either works, but ので is a bit more considerate.

Can I put the result clause first? Like “早く帰ります、用事があるので”?

Reason-first is most natural: 用事があるので、早く帰ります. If you want the result first, finish with a full sentence: 早く帰ります。用事があるからです (“It’s because I have something to do”). Don’t leave ので hanging after a period.

What does 早く mean here—“early,” “soon,” or “quickly”?

Here it means “early” (earlier than usual/expected). 早く often means “early” in time. If you mean speed (“quickly”), use 急いで or すぐ. 速く帰ります is uncommon; say 急いで帰ります or すぐ帰ります.

Why 早く and not 早い?

早い is the adjective (“early”), used before nouns (早い時間). 早く is the adverbial form (“early/soon”) that modifies verbs: 早く帰ります (“go home early”).

Why 帰ります and not 行きます or 戻ります?

帰る means “to return (to one’s home/base).” 行く is “to go (to some place),” not specifically home. 戻る is “to go back” to a previous place or state; it doesn’t imply “home.” For leaving work to go home, 帰ります is the natural verb.

How can non‑past 帰ります refer to the future?

Japanese non‑past covers both present and future. Context gives the future reading. If you want to emphasize intention, you could say 帰るつもりです or 帰りますね (depending on context).

Can I drop particles in casual speech?

Yes. A natural casual version is: 今日、用事あるので、早く帰る. Both は and が can drop in casual speech, especially inside subordinate clauses.

Who is the subject? Why isn’t “I” stated?

Japanese often omits obvious subjects. Here it’s understood as “I.” You can say 私は to clarify or contrast (私は今日は用事があるので…) but it’s usually unnecessary.

How would I make this more polite when talking to a boss?

For workplace politeness:

すみませんが、本日、所用がございますので、早退いたします。
申し訳ありません。今日は私用がございますので、失礼いたします。 Using 所用/私用 and ございます/いたします makes it more formal.

Are the spaces and comma used correctly?

Japanese normally doesn’t use spaces between words. Write it as: 今日は用事があるので、早く帰ります。 The comma (、) before the main clause is natural.