Learn Hardcore Japanese: natuyasumi ni nihongo wo benkyousimasu. - I study Japanese during summer vacation.

The particle に marks a specific point in time when an action occurs. Here, 夏休みに means “during summer vacation” or “in the summer break,” indicating when you will study Japanese.

Why is を used after 日本語 in 日本語を勉強します?

The particle を marks the direct object of a verb. 日本語を勉強します literally means “study Japanese,” where 日本語 (Japanese language) is what you are studying.

Can I replace に with は in 夏休みに日本語を勉強します?

You could say 夏休みは日本語を勉強します, but that changes the nuance. は turns 夏休み into the topic (“As for summer vacation, I will study Japanese”), whereas に simply indicates the time. Using は suggests contrast or emphasis, e.g. implying you do other things at other times.

Why is there no subject like 私 or 僕 in the sentence?

Japanese often omits subjects when they are obvious from context. In this case, it’s understood to mean “I will study Japanese during summer vacation,” so you don’t need to say 私は explicitly.

What is the typical word order in this sentence, and why does the time expression come first?

Standard Japanese word order is Subject–Object–Verb (SOV). Time expressions like 夏休みに commonly appear at the beginning for clarity:
(私は) 夏休みに日本語を勉強します。
Even if you move them, the verb must stay at the end.

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

勉強します is the polite form (ます-form), appropriate for most conversations. 勉強する is the plain (dictionary) form, used in casual speech or writing among friends.

How do you read the entire sentence in romaji?

It reads: Natsu yasumi ni nihongo o benkyō shimasu.