Learn Hardcore Japanese: kinou ane to issyo ni heya wo souzisimasita. - Yesterday I cleaned the room with my older sister.

Japanese often omits the subject when it’s clear from context. Here, because you’re talking about your own action (cleaning with your sister) and using a verb in polite past form (掃除しました), the listener automatically understands “I” as the subject. If you wanted to be explicit or contrast with someone else, you could add 私は at the beginning:
昨日、私は姉と一緒に部屋を掃除しました。

Why do we use と after 姉?

The particle と marks accompaniment or “with.”
– 姉と means “with (my) older sister.”
Without と, you can’t grammatically link 姉 to the action of cleaning.

If と already means “with,” why do we also have 一緒に?

– と indicates who you’re doing something with.
– 一緒に emphasizes that you did it together.
You can say 姉と掃除しました and it’s correct, but 姉と一緒に掃除しました stresses the joint effort: “I cleaned together with my sister.”

Why is 部屋 marked with を instead of は or が?

The particle を marks the direct object of a transitive verb.
– 部屋を掃除しました literally means “cleaned the room.”
If you used は (部屋は掃除しました), you’d be making “the room” the topic—implying contrast (“As for the room, I did clean it, but…”) or setting it up for further comment.

What kind of verb is 掃除しました?

掃除 is a noun meaning “cleaning.” The verb 掃除する (“to clean”) is a suru-verb. In polite past tense:
– 掃除する → 掃除します (polite present/future) → 掃除しました (polite past).

Why is 昨日 placed at the beginning of the sentence?

Time expressions like 昨日 (“yesterday”) customarily appear at or near the start of a Japanese sentence to set the temporal context. Japanese word order is flexible, but time-markers usually precede the verb and often the subject.

How do you pronounce 部屋, and is there any pitch-accent to note?

部屋 is pronounced へや (heya). Standard Tokyo pitch-accent is low-high: [hé-ya].

Can I omit 一緒に or と? How does that change the nuance or grammaticality?

– Omitting と is ungrammatical: you need と to link 姉 (“older sister”) to the action.
– Omitting 一緒に (姉と掃除しました) is perfectly grammatical and still means “I cleaned with my sister,” but without the extra emphasis on doing it together as a unit.