Learn Hardcore Japanese: watasi ha bousi no iro wo uwagi ni awasemasu. - I match the color of my hat to my jacket.

Using が (私が帽子の色を上着に合わせます) is grammatically possible but changes the focus. が would highlight I specifically as the one who does it (for example, contrasting with someone else). In a neutral statement about your own habit, 私は is more natural.

In conversation, 私 is often omitted altogether if it’s obvious from context (see next question).

Can I omit 私? Would 帽子の色を上着に合わせます still be correct?

Yes. 帽子の色を上着に合わせます。 is perfectly natural.

Japanese often omits the subject when it’s clear from context. In many real-life situations (talking about what you do with clothes), it would be more common to drop 私 and just say the rest. The meaning would still be understood as I match the color of the hat to the jacket.

Why is it 帽子の色 (color of the hat) and not just 帽子 (the hat) as the object?

Because the verb 合わせます here is specifically about matching colors (or some property), not physically putting the hat and jacket together.

帽子の色を上着に合わせます。
→ I match the *color of my hat to my jacket.* (I choose a hat color that suits the jacket.)

If you said:

帽子を上着に合わせます。

it can still be understood as I match my hat to my jacket, but it sounds a bit vague about what aspect you’re matching. Using 帽子の色 makes it explicit that we’re talking about color.

What exactly does の do in 帽子の色?

の here links two nouns into a possessive/attributive relationship: hat → color:

帽子の色 = the color of the hat
Pattern: A の B = B of A / A’s B

Compare:

帽子の色 – the hat’s color (focus on color)
色の帽子 – a colored hat / a hat of a certain color (focus on hat, described by its color)

In your sentence, we care about the color itself as the thing we’re adjusting, so 帽子の色 is the natural choice.

What is the role of を and に with 合わせます? Which noun is being matched to which?

With 合わせる in this meaning, the pattern is:

X を Y に合わせる
= to match/adjust X to Y
(X is changed/selected so that it suits Y)

So in:

帽子の色を上着に合わせます。
- 帽子の色 (hat color) = X, the thing being adjusted → marked with を
- 上着 (jacket) = Y, the reference/standard → marked with に

English also has both directions (match A to B / match B with A), so paying attention to を/に tells you which side is being made to fit the other.

Could I say 上着と合わせます instead of 上着に合わせます? What’s the difference between に and と here?

Both are possible, but the nuance is slightly different:

上着に合わせます
→ I match (it) *to the jacket.
The jacket is the *standard, and something else (hat color) is adjusted to fit it.
上着と合わせます
→ I match (it) *with the jacket.*
This sounds more like “coordinate with” or “goes together with” the jacket, often used more generally about coordinating items to wear together.

In your specific sentence where you clearly have X を Y に合わせる, に is the best fit because you’re saying “adjust X so it fits Y.”

Can I change the word order? For example, is 上着に帽子の色を合わせます OK?

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

These are both grammatical and mean the same thing:

帽子の色を上着に合わせます。
上着に帽子の色を合わせます。

The difference is subtle in emphasis: whatever comes earlier can feel slightly more topical or foregrounded. But there is no meaning change like in English word order; the particles を and に still mark their functions clearly.

What form and politeness level is 合わせます? How is it related to 合わせる?

合わせます is the polite, non-past form (often called the “-ます form”) of the verb 合わせる.

Dictionary/plain form: 合わせる
Polite non-past: 合わせます (present or future, depending on context)
Polite past: 合わせました (matched)
Plain non-past: 合わせる (match / will match)

So 合わせます here can be understood as I (generally) match / I will match depending on context.

What’s the difference between 合わせます and 合います?

They are related but different verbs:

合わせる (他動詞, transitive) – to match/adjust something to something
Pattern: X を Y に合わせる
→ I match X to Y (I actively choose/change X)
合う (自動詞, intransitive) – to match / go well (by itself)
Pattern: X が Y に合う
→ X matches Y / X goes well with Y

Your sentence uses the transitive version (合わせます) because you are actively matching one thing to another.

If you want to say it just matches (as a state), you’d use 合う, for example:

How would I say “The color of my hat matches my jacket” (as a state, not an action)?

You’d typically use 合う and often the -ている form for a current state:

帽子の色は上着に合っています。
= The color of my hat matches my jacket / goes well with my jacket.

Structure:

帽子の色は – topic (hat color)
上着に – reference item (jacket)
合っています – is matching / goes well (literally is in a state of matching)

How would I say “I match my jacket to my hat” instead? (reverse the direction)

Just swap which item is marked with を (thing adjusted) and which with に (standard):

上着の色を帽子に合わせます。
= I match the color of my jacket to my hat.

Here:

上着の色 (jacket color) is now the thing being adjusted → を
帽子 (hat) is the standard → に

Does the sentence explicitly say “my” hat and “my” jacket?

No. Japanese usually omits possessive pronouns like my / your / his unless you really need to clarify ownership.

私は帽子の色を上着に合わせます。

Literally: As for me, [I] match the color of the hat to the jacket.

From context (you talking about your own clothes), listeners automatically understand it as my hat and my jacket, even though 私の or 僕の is not written. You’d only add 私の帽子 or 私の上着 if the owner might be ambiguous.