Learn Hardcore Japanese: watasi ha tomodati to issyo ni nihongo wo benkyousimasu.

私 (watashi) means “I” and marks the topic of the sentence. In Japanese you often drop the pronoun if the context is clear. So instead of
私は友達と一緒に日本語を勉強します。
you can simply say
友達と一緒に日本語を勉強します。
and it will still mean “I study Japanese together with friends.”

What is the function of the particle は?

は (wa) is the topic-marker particle. It sets 私 (“I”) as the theme of the sentence: “As for me…”. It does NOT mark the grammatical subject as strictly as English does; it highlights what the sentence is talking about.

What does the particle と after 友達 mean?

Here と (to) means “with” when used after a noun. 友達と therefore means “with friends” or “together with a friend.”

What is the role of 一緒に, and why does it end in に?

一緒に (issho ni) is an adverbial phrase meaning “together.” Historically it’s a noun (一緒) plus the particle に to turn it into an adverb. In practice you treat 一緒に as a single unit: “together.”

Is 一緒に necessary? What’s the difference between 友達と日本語を勉強します and 友達と一緒に日本語を勉強します?

Grammatically you can drop 一緒に because 友達と already implies doing something with friends.
• 友達と日本語を勉強します。
However, adding 一緒に emphasizes the idea of “doing it together,” making the action more vivid or explicit.

Why is を used after 日本語?

を (o) marks the direct object of a transitive verb. 日本語を勉強します literally means “(I) study the Japanese language,” and を shows that 日本語 is what you’re studying.

What form is 勉強します, and how do I change it to plain or past tense?

勉強します is the polite non-past (dictionary form + ます) of the verb 勉強する (“to study”).
• Plain non-past: 勉強する (benkyō suru) – “study”／“will study”
• Polite past: 勉強しました (benkyō shimashita) – “studied”
• Plain past: 勉強した (benkyō shita) – “studied”

Is this sentence present or future tense? How do I say “I will study”?

Japanese non-past tense (the –ます or dictionary form) covers both present and future, depending on context.
• Without a time expression, 勉強します can mean “I study” or “I will study.”
• To make it clearly future you can add a time phrase:
明日、友達と一緒に日本語を勉強します。
“Tomorrow I will study Japanese together with friends.”

Can I reorder the word groups, for example putting 日本語を before 友達と一緒に?

Yes. Japanese word order is relatively flexible so long as particles stay attached to their words and the verb ends the sentence. For instance:
日本語を友達と一緒に勉強します。
This still means “I study Japanese together with friends,” though native speakers often choose an order based on emphasis or rhythm.