Learn Hardcore Japanese: watasi ha haha to natuyasumi ni issyo ni ryokou wo suru to yakusokusimasita. - I promised my mother that we would travel together in the summer vacation.

Xと can mean:
- “with X / together with X” (companion)
- “and X” (listing)
- It can also mark a partner in an action (e.g. 彼と結婚する “to marry him”).

In this sentence:

母と夏休みに一緒に旅行をする
is understood as
“to go on a trip together with my mother during the summer vacation.”

So 母と is attached to 旅行をする (the traveling), not to 約束しました.

If you want to say “I promised my mother (i.e. promised to her)”, you’d typically use:

母に約束しました。 = “I promised my mother.”
母と約束しました。 = “I made a promise with my mother / We promised each other.” (focuses on both being parties to the promise)

Why is there に after 夏休み? What does 夏休みに mean exactly?

夏休みに marks a time when something happens.

に after a time word (day, date, period, etc.) often means “on / at / in / during”.
- 月曜日に = on Monday
- ３時に = at 3 o’clock
- 夏休みに = during the summer vacation / in the summer vacation

So 夏休みに旅行をする = “to travel during the summer vacation.”

There are two に’s: 夏休みに and 一緒に. Are they the same に?

Grammatically it is the same particle に, but the usage feels different:

夏休みに – time marker
Marks when the action happens: “in/during summer vacation”.
一緒に – makes 一緒 (“together”) into an adverb, “together(ly)”.
Many adverb-like words use に:
- 静かに (quietly)
- ゆっくりに (slowly – though ゆっくり alone is more common)
- 一緒に (together)

So:

夏休みに = “in summer vacation” (time)
一緒に = “together” (manner)

They’re the same particle, but serving different roles in the sentence.

Why is it 旅行をする and not just 旅行する? Are both correct?

Both 旅行をする and 旅行する are grammatically correct and common.

旅行 is originally a noun meaning “trip / travel”.
旅行をする literally = “to do travel”.
Over time, 旅行する has become a standard verb form.

Nuance:

旅行をする can feel a bit more explicit or sometimes slightly more formal in some contexts.
旅行する is very natural and common in everyday speech.

In this sentence, you could say:

母と夏休みに一緒に旅行をすると約束しました。
母と夏休みに一緒に旅行すると約束しました。

Both sound natural. Learners often default to N + をする as a safe pattern.

What is the と between する and 約束しました doing?

That と is the quotation particle, marking the content of the promise.

Pattern:

[clause] と約束する
= “to promise that [clause]”

So:

母と夏休みに一緒に旅行をすると約束しました。
= “(I) promised that (I) would travel together with my mother during summer vacation.”

This と works like in:

「行きます」と言いました。 – “(He) said, ‘I’ll go.’”
明日行くと決めた。 – “I decided to go tomorrow / I decided that I’ll go tomorrow.”

Don’t confuse this と with the earlier 母と:

母と = with my mother
[旅行をする] と約束しました = promised that (I) will travel

約束しました is past tense, but the trip is in the future. Why?

In Japanese, the tense applies to each verb separately.

約束しました = “promised” (the act of promising happened in the past)
旅行をする uses the non-past form, which in Japanese covers both present and future.

So the structure is:

(Past) I promised
(Non-past content) that I will travel

This matches natural English too:

“I promised I would go.”
“I said I will go.”

The promise is in the past, but the trip is understood as a future action.

Can I omit 私は? Would the sentence still be natural?

Yes, very natural.

Japanese often omits subjects when they are clear from context. You can say:

母と夏休みに一緒に旅行をすると約束しました。

A native speaker will usually understand this as:

“I promised to go on a trip with my mother during summer vacation.”

You include 私は when:

You need to introduce who the topic is.
You want to contrast yourself with others:
私は行きますが、弟は行きません。
→ “I will go, but my younger brother won’t.”

Can I change the word order? For example, is 夏休みに母と一緒に旅行をすると約束しました okay?

Yes. Japanese word order is relatively flexible as long as the particles stay attached to the right words.

These are all natural:

私は母と夏休みに一緒に旅行をすると約束しました。
私は夏休みに母と一緒に旅行をすると約束しました。
母と夏休みに一緒に旅行をすると約束しました。
夏休みに母と一緒に旅行をすると約束しました。

What you shouldn’t break is tight units like:

母と一緒に (together with my mother)
旅行をする (to travel)

As long as those stay intact and the particles (と, に, を) stay on the right nouns, moving time expressions (夏休みに) earlier or later is fine and just slightly shifts emphasis.

What is the difference between 母, お母さん, and 私の母 here? Which is most natural?

All can appear, but they have different uses and levels of politeness.

母
- Neutral, plain word for “my mother”.
- Used when talking about your own mother to outsiders (not to your mother herself).
- Polite enough in most contexts.
私の母
- Literally “my mother”.
- A bit more explicit; may be used when you need to be very clear it’s your mother (e.g., introducing her).
- Often, 私の is dropped because 母 already implies “my mother” in context.
お母さん
- Polite form meaning “mother”, but:
  - Used to address your own mother: お母さん、聞いて。 (“Mom, listen.”)
  - Or to refer to someone else’s mother politely: 田中さんのお母さん.
- You usually don’t call your own mother お母さん when talking about her to strangers; you say 母 instead.

In this sentence, 母 is the natural choice unless context requires something more explicit like 私の母.

If 母と already means “with my mother,” is 一緒に (“together”) really necessary?

母と by itself can mean “with my mother,” so:

母と旅行をする is already understandable as “travel with my mother.”

However, 母と一緒に is a very natural and common collocation:

母と一緒に旅行をする
literally “to travel together with my mother.”

Nuance:

母と旅行をする: simple “travel with my mother.”
母と一緒に旅行をする: emphasizes the togetherness a bit more, and sounds very natural in conversation.

So 一緒に is not grammatically required, but it is very commonly added and sounds friendly and clear.

Why is 旅行 marked with を even though it’s an activity, not a physical object?

In Japanese, many action nouns combine with する and take を:

勉強をする – to do study
運動をする – to do exercise
掃除をする – to do cleaning
旅行をする – to do travel / to travel

Here, を marks what is being “done” by する. It doesn’t have to be a concrete object; it can be an abstract activity.

Over time, many of these have fused into single verbs:

勉強する, 運動する, 掃除する, 旅行する, etc.

So 旅行をする and 旅行する are structurally a bit different (noun + する vs. verb), but both are perfectly natural.