hito ga ooi hi ha, tikadou no hou ga tizyou yori arukiyasui desu.

Japanese puts modifying clauses before the noun they describe.

So 人が多い日 literally works like:

• 人が多い = there are many people
• 日 = day

Together, it means a day when there are many people or a busy/crowded day.

Unlike English, Japanese does not need a word like when or that here. The clause simply goes in front of the noun.

Here, が marks 人 as the subject of the small clause 人が多い.

That clause is just describing the day: the people are many / there are many people.

Using は inside this kind of neutral modifying clause is less natural, because は tends to mark a topic or contrast. In this sentence, 人が多い is just factual description, so が is the normal choice.

The は marks the whole phrase 人が多い日 as the topic.

So:

• 人が多い日 = days when there are many people
• 人が多い日は = as for days when there are many people...

In natural English, this often becomes On busy days, ... or When there are lots of people, ...

So this は is setting the situation or context for the rest of the sentence.

This uses a very common comparison pattern:

X のほうが Y より adjective

= X is more adjective than Y

In this sentence:

• X = 地下道
• Y = 地上
• adjective = 歩きやすい

So the structure means:

The underground passage is easier to walk on than above ground.

ほう literally means something like side, direction, or option.

After a noun, N のほう often means the N side or the N option.

So:

• 地下道のほう = the underground-passage side/option

In comparisons, のほうが is extremely common and often just means the one that is more...

So here, 地下道のほうが means the underground passage is the one that is easier.

Because in the comparison pattern X のほうが Y より adjective, が marks the item being judged as more of that adjective.

So:

• 地下道のほうが = the underground passage is the one that is more...
• 地上より = than ground level / than above ground

This is just the standard grammar of comparison.

You could also make a different sentence with は, such as:

• 地下道は地上より歩きやすいです。

But with のほう, が is the normal pattern.

Japanese often leaves out repeated words when the meaning is clear.

Here, 地上より means than above ground or than the above-ground route. The listener understands that 地上 is the comparison target because 地下道 was already mentioned.

So the sentence does not need to repeat the whole idea explicitly.

歩きやすい is made from:

• 歩く = to walk
• verb stem 歩き
• やすい = easy to do

So:

• 歩きやすい = easy to walk / easy to walk on

This is a common pattern:

• 読みやすい = easy to read
• 食べやすい = easy to eat
• 使いやすい = easy to use

The opposite is 〜にくい:

• 歩きにくい = hard to walk on

です can also be added after an い-adjective to make the sentence polite.

So:

• plain: 歩きやすい
• polite: 歩きやすいです

Important point: the です here is not changing it into a noun. 歩きやすい is already a complete adjective.

That is why:

• 歩きやすいです = correct
• 歩きやすいだ = incorrect

Yes, 人の多い日 is also possible.

Both can mean something like a day with lots of people.

A rough difference is:

• 人が多い日: sounds like a straightforward clause, a day when there are many people
• 人の多い日: more like a descriptive noun phrase, often heard in expressions meaning crowded or full of people

In many real situations, the meanings are very close.

Japanese often omits things that English requires.

In English, we might say:

• It is easier to walk...

But Japanese does not need a dummy subject like it. Instead, it can simply name the thing being discussed:

• 地下道のほうが歩きやすいです

That already tells you what has the property easy to walk.

Yes, somewhat.

A very natural alternative is:

• 人が多い日は、地上より地下道のほうが歩きやすいです。

This may feel slightly easier to process because 地上より comes right before the thing being compared.

Japanese word order is flexible as long as the particles make the relationships clear. The core comparison markers are what matter:

• X のほうが
• Y より

So small rearrangements are possible without changing the basic meaning.