Learn Hardcore Japanese: otouto to issyo ni eiga wo mimasu. - I watch a movie with my younger brother.

と marks the companion (“with”), telling you who you’re doing something with—here, “with my younger brother.” 一緒に means “together,” functioning as an adverb that describes how you do the action. Put together, 弟と一緒に literally means “together with my younger brother.”

What exactly does 一緒に do in this sentence?

一緒に turns the idea of “together” into an adverbial phrase, modifying the verb 見ます (“watch”). Without it, you’d lose the nuance that you’re watching the movie together, rather than you watching it and your brother doing something else.

Why is there an に after 一緒?

In Japanese, 一緒 on its own is a noun meaning “togetherness.” To make it into an adverb (so that it can modify a verb), you add the particle に, giving 一緒に.

Why is the particle を used before 見ます?

を marks the direct object of a transitive verb. Here, 映画 (“movie”) is what you are watching, so it takes を to show that 映画 is the object of 見ます.

Can I drop the を and just say 弟と一緒に映画見ます?

Yes—in casual spoken Japanese, omitting を is common (映画見ます). In formal writing or polite speech, however, it’s better to include を to follow standard grammar.

Why is the verb 見ます at the end of the sentence?

Japanese follows a Subject–Object–Verb (SOV) order. Verbs always come at the end, after any objects (映画を) and adverbs (一緒に).

There’s no “I” in the sentence. Why?

Japanese often omits the subject when it’s clear from context. Here, if it’s understood that you’re the one speaking, you don’t need to say 私は (“I”).

Could I say 弟と映画を一緒に見ます instead?

You could, but it sounds less natural. Adverbs like 一緒に usually come right before the verb, so 弟と一緒に映画を見ます is the preferred, more fluid order.

How would I change this sentence to casual/dictionary form?

Switch 見ます to its dictionary form 見る:
弟と一緒に映画を見る。
This is suitable for casual conversation or writing.