kahun ga ooi hi ha, masuku wo site dekakemasu.

Because 花粉が多い is a clause modifying 日.

• 花粉が多い日 = a day when pollen is abundant / a high-pollen day
• Inside that clause, 花粉 is the subject of 多い, so が is the natural particle.

Then the whole phrase 花粉が多い日 becomes the topic of the sentence, so you get:

• 花粉が多い日は、…

In other words:

• が marks the subject inside the description
• は marks the whole day phrase as the topic

Using 花粉は多い日 would sound unnatural here.

This is a noun-modifying clause.

Japanese often puts a whole clause before a noun to describe it:

• 花粉が多い日 = day(s) on which pollen is high
• literally: pollen is many/abundant day

The structure is:

• 花粉 = pollen
• が = subject marker
• 多い = many / abundant
• 日 = day

So 花粉が多い directly modifies 日.

This is very common in Japanese:

• 雨が降る日 = a day when it rains
• 忙しい日 = a busy day
• 時間がない人 = a person who has no time

Because 多い is an い-adjective, and い-adjectives can directly modify nouns.

So:

• 多い日 = a day that has a lot of something / a day when something is abundant

You do not say:

• 多いの日 ✘

Compare:

• 暑い日 = a hot day
• 忙しい人 = a busy person
• 花粉が多い日 = a day when pollen is abundant

The particle の is used with nouns, not with い-adjectives in this kind of structure.

多い is describing the amount of 花粉.

So 花粉が多い means:

• there is a lot of pollen
• pollen levels are high

It does not mean that the day itself is many. The whole clause 花粉が多い describes what kind of day it is.

So:

• 花粉が多い日 = a day on which there is a lot of pollen

The は marks 花粉が多い日 as the topic of the sentence.

So the sentence is structured like:

• As for days when pollen is high, I wear a mask and go out.

This is a common Japanese pattern:

• 暑い日は、水をたくさん飲みます。
• 暇な日は、家で休みます。

The speaker is saying what they do on such days.

して is the て-form of する.

In Japanese, マスクをする means to wear a mask.
So:

• マスクをして = wearing a mask / put on a mask, and…

Then it connects to the next verb:

• マスクをして出かけます = I wear a mask and go out

This て-form often links actions, but here it can also imply the state of doing the next action while wearing the mask.

Other similar examples:

• 眼鏡をして出かける = go out wearing glasses
• 帽子をかぶって出かける = go out wearing a hat

Japanese uses different verbs depending on what is being worn.

For masks, マスクをする is the standard expression.

Some common patterns:

• 帽子をかぶる = wear a hat
• 服を着る = wear clothes
• 靴を履く = wear shoes
• 眼鏡をかける = wear glasses
• マスクをする = wear a mask

So although English often uses just wear, Japanese chooses different verbs depending on the item.

出かけます means to go out, often with the nuance of leaving home or one’s usual place.

行きます simply means to go.

So here, 出かけます fits well because the idea is:

• on high-pollen days, I put on a mask before going out

If the sentence used 行きます, it would sound more like going to a destination, while 出かけます emphasizes the act of going out.

It can suggest both, but in natural English the idea is usually:

• go out wearing a mask

The て-form can connect actions in sequence, but with clothing/accessories it often describes the condition in which the next action happens.

So here the sense is not just:

• first I put on a mask
• then I go out

It is more naturally:

• I go out with a mask on

Japanese nouns usually do not mark singular vs. plural the way English does.

So 花粉 can simply mean pollen as a substance. You do not need a plural ending.

That is why 花粉が多い naturally means:

• there is a lot of pollen
• pollen levels are high

Japanese focuses on quantity through 多い, not by changing the noun.

The comma is mainly for readability and a natural pause.

• 花粉が多い日は、マスクをして出かけます。

Since the topic phrase at the beginning is a little long, the comma helps separate it from the main statement.

It is not a special grammar point, and Japanese punctuation is often more flexible than English punctuation. The sentence could sometimes appear without a comma too, especially in casual writing.

A casual version would be:

• 花粉が多い日は、マスクをして出かける。

The main change is:

• 出かけます → 出かける

This changes the sentence from polite to plain style.

You might also hear other casual variations depending on context, but this is the straightforward plain-form equivalent.

Yes, sometimes, but the nuance changes a little.

• 花粉が多い日は = as for days when pollen is high
• 花粉が多い日には = on days when pollen is high / especially on such days

Adding に makes the time expression a bit more explicit.
Both are possible, but 日は is very natural and common here.

It sounds like a habitual action or a general rule.

Because the sentence is in non-past polite form and has no specific date or time, it usually means something like:

• this is what I do on high-pollen days

If the speaker wanted to talk about one specific occasion, the sentence would usually include more context, such as 今日 or a past form.