Learn Hardcore Japanese: natuyasumi ni minami no umi de oyogimasu. - I swim in the southern sea during summer vacation.

The particle に marks the point in time when an action takes place. Here 夏休み (summer vacation) is the time frame, so 夏休み＋に means “during/in summer vacation.” Without に, it wouldn’t be clearly defined as a time expression.

Why is there a の between 南 and 海?

The particle の links two nouns in a modifier relationship. 南 means “south” and 海 means “sea.” By saying 南の海, you’re literally saying “the sea of the south,” i.e. “southern sea.” It’s the same pattern as 日本の車 (“Japanese car”).

Why is there a で after 海 instead of another particle?

The particle で indicates the location where an action occurs. Since 泳ぎます (to swim) is an action happening in the sea, you use 海で. If you used another particle like を, it would imply the sea is a direct object, which doesn’t fit with how 泳ぐ works here.

Why is the subject (like “I”) omitted in the sentence?

Japanese often drops the subject when it’s clear from context. If you’re talking about yourself, there’s no need to say 私は. Listeners infer who’s performing the action. Omitting the subject makes the sentence more natural and less repetitive.

Could I use 夏休み中に instead of 夏休みに?

Yes, you can say 夏休み中に南の海で泳ぎます.

夏休みに treats “summer vacation” as a point in time.
夏休み中に emphasizes the entire period of summer vacation.
Both are grammatically correct; the nuance is just slightly different.

Why is the word order Time – Location – Verb, rather than “I swim in the sea during summer vacation”?

Japanese typically follows Subject–Object–Verb (SOV) order and places time and location adverbials before the verb. So you get:

Time expression (夏休みに)
Location expression (南の海で)
Verb (泳ぎます)
Even if the subject is omitted, the time–location–verb ordering stays the same.