sono aoi bousi mo ii desu ga, kono akai bousi ni simasu.

その and この are both demonstratives, but they point to things from different perspectives.

• この = this thing, near the speaker
• その = that thing, near the listener or already identified in the conversation

So in:

• その 青い 帽子 = that blue hat
• この 赤い 帽子 = this red hat

The speaker is contrasting two hats and treating the red one as the one closer to them or the one they are currently choosing.

A useful set to remember is:

• この = this
• その = that
• あの = that over there

In Japanese, adjectives usually come before the noun they describe, just like in English.

So:

• 青い 帽子 = blue hat
• 赤い 帽子 = red hat

Here, 青い and 赤い are i-adjectives, so they can directly modify a noun without any extra word.

Compare:

• 青い 帽子 = a blue hat
• いい 帽子 = a good hat

This is very normal Japanese word order.

いい is the normal everyday form meaning good. Historically, it comes from よい, and both exist, but いい is much more common in conversation.

So:

• いいです = is good / okay
• よいです = also correct, but more formal or less common in casual speech

One thing to watch out for: in some conjugations, よい shows up again.

For example:

• よくない = not good
• not いくない

So learners often need to memorize that いい is a little irregular.

Here, も means also or too.

So その 青い 帽子 も いいです means:

• That blue hat is also good
• or more naturally: That blue hat is nice too

The nuance is that the speaker is acknowledging the blue hat positively, but then choosing something else.

It sounds like:

• That blue hat is nice too, but...

So も helps set up the contrast with the second half of the sentence.

Great question. が does often mark the subject, but here it is doing a different job.

In this sentence, が means something like:

• but
• although

So:

• いいですが = it is good, but...

This is a very common polite pattern for soft contrast.

So the sentence structure is:

• その 青い 帽子 も いい です が、
• That blue hat is nice too, but...

This use of が often sounds softer and more polite than a direct でも in similar contexts.

にします comes from the pattern X にする, which means:

• to decide on X
• to choose X
• to make it X

In shopping or ordering situations, it often means:

• I’ll take X
• I’ll go with X
• I choose X

So:

• この 赤い 帽子 にします means
• I’ll choose this red hat
• I’ll go with this red hat

This is a very common expression when deciding between options.

Examples:

• コーヒーにします。 = I’ll have coffee.
• これにします。 = I’ll take this one.

Because this is the fixed grammar pattern X にする.

In this pattern, に marks the thing you are deciding on as the result of the choice.

So:

• この 赤い 帽子 にします = I choose this red hat / I’ll make it this red hat

It is not the ordinary direct-object pattern here.

Compare:

• 帽子を買います = I buy a hat
• この帽子にします = I decide on this hat

So にする is best learned as a set expression meaning choose / settle on.

Yes, Japanese could leave it out if the meaning is obvious.

For example, someone could say:

• その 青い の も いい ですが、この 赤い の にします。

or even more simply in context:

• その 青い の も いい ですが、こっち にします。

But repeating 帽子 makes the sentence clearer and easier for learners. It also sounds perfectly natural, especially when you want to clearly contrast two specific hats.

So repetition here is not strange. It is just explicit.

Yes. Japanese often leaves out the subject when it is obvious from context.

In English, we would naturally say:

• I’ll choose this red hat.

But in Japanese, the I is omitted because it is understood from the situation. The speaker is obviously the one making the choice.

So the sentence does not need:

• わたしは

unless the speaker wants extra emphasis or contrast.

It is polite standard Japanese.

You can tell because of:

• です
• します

These are polite forms.

So this would work well in situations like:

• talking to a store clerk
• speaking politely to someone you do not know well
• making a polite choice in conversation

A more casual version might be:

• その青い帽子もいいけど、この赤い帽子にする。

That sounds less formal and more conversational.

Not strongly rejecting it. The nuance is softer than that.

The sentence suggests:

• The blue hat is nice too
• but I’m going to choose the red one

So the speaker is acknowledging the blue hat positively before giving their decision. This can sound polite, thoughtful, or less blunt than simply saying:

• I don’t want the blue one.

So the feeling is more like:

• That blue one is good too, but I’ll go with this red one.

Yes, but the nuance would change.

Compare:

• その 青い 帽子 も いいです が…

  • That blue hat is good too, but...
  • implies another option is also good

• その 青い 帽子 は いいです が…

  • As for that blue hat, it is good, but...
  • sounds more like setting up a contrast topic

With も, the speaker sounds like they are saying both hats are good, but they prefer the red one.

With は, the contrast can feel a bit stronger or more structured.

In this sentence, も is very natural because the speaker is politely acknowledging the blue hat before choosing the red one.

You can do a rough word-by-word breakdown, but the most natural English translation will be smoother.

Breakdown:

• その = that
• 青い = blue
• 帽子 = hat
• も = also / too
• いい = good
• です = polite copula
• が = but
• この = this
• 赤い = red
• 帽子 = hat
• に = for the choice / as
• します = do / decide

Very literal:

• That blue hat too is good, but I will make it this red hat.

Natural English:

• That blue hat is nice too, but I’ll choose this red hat.
• That blue one is good too, but I’ll go with this red hat.

So literal translation helps you see the grammar, but natural translation is better for actual meaning.