Learn Hardcore Japanese: sono bousi ha watasi no huku ni au.

Using は makes その帽子 the thing we’re talking about, rather than just the grammatical subject in a neutral way. It can also set up contrast, like “That hat (as opposed to other hats) matches my clothes.”

What does の do in 私の服?

の is a possessive/connecting particle.

私の服 literally means “my clothes”:

私 – I / me
の – of / ’s
服 – clothes, clothing

Pattern: Noun1 の Noun2 often means “Noun2 that is related to Noun1”, very often possession:

彼の本 – his book
先生の車 – the teacher’s car
日本の文化 – Japanese culture (culture of Japan)

What exactly is the role of に in 服に合う?

With 合う in the “match/suit” sense, に marks the thing that something matches / goes well with.

Pattern: X は Y に合う
= “X goes well with Y / X matches Y.”

In this sentence:

その帽子は – that hat
私の服に – with my clothes
合う – matches / goes well

So に here marks the target or basis of comparison: what the hat is matching to.

Other examples:

このワインは魚料理に合う。 – This wine goes well with fish dishes.
赤いネクタイはこのスーツに合わない。 – A red tie doesn’t go with this suit.

What does 合う mean here, and what form is it in?

Here 合う means “to match / to go well with / to suit (in combination)”.

It’s in dictionary (plain, non-past) form:

Verb: 合う (godan verb)
Non-past plain: 合う
Non-past polite: 合います
Past plain: 合った
Negative plain: 合わない

Other common meanings of 合う (depending on context) include to fit, to meet, to come together, etc., but with clothes/food/colours it’s usually “to match / to go well with.”

Why isn’t there です at the end, like 合います or 合うんです?

です is not attached to verbs in Japanese; it’s the copula used mainly with nouns and adjectives (X is Y). Verbs already contain tense and politeness information.

合う is a verb, so it can stand alone in plain style.
To make it polite, you change the verb itself: 合う → 合います.

So:

その帽子は私の服に合う。 – plain, casual
その帽子は私の服に合います。 – polite

You don’t need です after 合う; it would be ungrammatical on its own (合うです ✕). You might see 合うんです or 合うのです, but that’s a different structure adding explanation/emphasis, not just “合う + です” mechanically.

Can I use が instead of は, like その帽子が私の服に合う?

Yes, その帽子が私の服に合う is grammatically correct, but the nuance changes a bit.

その帽子は私の服に合う。
– Topic-comment: “That hat, it matches my clothes.”
– More like you’re already talking about that hat and describing it.
その帽子が私の服に合う。
– が marks その帽子 as the (grammatical) subject and tends to sound like:
“It’s that hat that matches my clothes” (often as new information, or in contrast to other possibilities).

In simple statements, both can often be translated the same in English, but:

は: “as for that hat…” (topic)
が: “that hat (specifically) is the one that…” (subject, often focus/new info)

Can I change the word order, like 私の服にその帽子は合う?

Yes, that is grammatically possible. Japanese word order is relatively flexible as long as particles are correct.

その帽子は私の服に合う。 (more neutral, common)
私の服にその帽子は合う。 (puts a bit more emphasis on 私の服に – “With my clothes, that hat works.”)

Both say the same basic thing, but:

Keeping the は-phrase (その帽子は) near the beginning is the most typical, natural order.
Moving parts around can slightly change what feels emphasized or contrasted.

What exactly does 服 mean here? Is it singular or plural?

服 means clothes / clothing / outfit in general.

Japanese nouns typically don’t mark singular vs plural; context tells you.

So 私の服 can mean:

my clothes
my outfit
the clothes I’m wearing

In English we choose singular or plural, but Japanese just uses 服. If you want to explicitly refer to multiple items, you might add something like 何着も服 (many pieces of clothing), but plain 服 is neutral.

Why isn’t there a particle after 私?

Because 私の服 is a single noun phrase: “my clothes”.
私の is just a possessive modifier before 服.

私
- の
  - 服
    = I + ’s + clothes
    = my clothes

Particles like は, が, に, を attach to whole noun phrases, not to individual words inside them. So you’d put a particle after 私の服, not after 私:

私の服に合う – matches my clothes
私に合う – suits me (here に is after 私, because 私 is the noun being marked)

What is the difference between 合う and 似合う with clothes?

Both can appear in clothing contexts, but they focus on different things:

合う – “to match / to go well (together)”
Focus: how two things go together (colours, styles, items).
- その帽子は私の服に合う。
  – That hat matches my clothes. (Hat + clothes go well together.)
似合う – “to suit / to look good on (someone)”
Focus: how something suits a person.
- その帽子は私に似合う。
  – That hat suits me / looks good on me.

So:

Use 合う for combinations of items (hat + clothes, wine + food, colours, etc.).
Use 似合う when talking about whether something looks good on a person.

Could I say その帽子は私の服と合う instead of 服に合う?

The most natural patterns are:

X は Y に合う – “X goes well with Y.”
X と Y は（よく）合う – “X and Y (really) go well together.”

So you’d normally say:

その帽子は私の服に合う。 – That hat goes with my clothes.
その帽子と私の服はよく合う。 – That hat and my clothes go well together.

X は Y と合う (your version) is not the most typical pattern; you’re more likely to hear it in the “X と Y は合う” structure with both items tied together by と. For clear, natural Japanese, stick with X は Y に合う or X と Y は合う.

What is the nuance of その compared with この and あの in this sentence?

Japanese has a three-way “this/that” system:

この – “this … (near me, the speaker)”
その – “that … (near you, the listener, or already mentioned)”
あの – “that … over there (away from both speaker and listener)”

In その帽子, using その suggests:

the hat is closer to the listener, or
you both already know which hat is being referred to (it’s been mentioned or is obvious from context).

So nuances:

この帽子 – this hat (near me / I’m holding it)
その帽子 – that hat (near you, or that one we’re talking about)
あの帽子 – that hat over there / that (distant) hat

Why are there spaces between each word? Aren’t Japanese sentences normally written without spaces?

Yes, in normal Japanese writing there are no spaces between words. You’d typically see:

その帽子は私の服に合う。

The spaces in その　帽子　は　私　の　服　に　合う。 are a teaching aid used in textbooks and classrooms to help learners see word boundaries and particle positions.

So:

With spaces: good for learners.
Without spaces: how native Japanese is actually written.

What tense/aspect does 合う express here? Is it “matches”, “will match”, or “is matching”?

Plain 合う is non-past. Japanese non-past covers both:

present / general facts, and
future events.

In this sentence, depending on context, it can be understood as:

“That hat matches my clothes (in general).”
“That hat will match my clothes.”

Japanese doesn’t distinguish “matches” vs “is matching” vs “will match” as strictly as English; non-past 合う can cover all of these, with the exact nuance inferred from context.