gongwone mogiga manhaseo darireul du beonina mullyeosseoyo.

Because this clause is describing where mosquitoes exist / are numerous, not where an action is being performed.

A very common pattern is:

• place + 에 + noun + 이/가 많다
• 공원에 모기가 많다 = there are a lot of mosquitoes in the park

By contrast, 에서 is more often used for the place where an action happens, like eat, study, play, work.

So here 공원에 is the natural choice.

Because 모기 is the thing that is being described as many.

In Korean, with words like 많다 and 적다, the noun that is numerous or few typically takes 이/가:

• 사람이 많아요 = there are many people
• 차가 적어요 = there are few cars

So:

• 공원에 모기가 많아서 = because there were many mosquitoes in the park

Korean often leaves nouns unmarked for plural when the meaning is already clear.

Here, 많다 already tells you there are many of them, so 모기 naturally means mosquitoes in context.

• 모기가 많아요 = there are many mosquitoes

Using 모기들 is not impossible, but it is usually unnecessary here and can sound heavier than needed.

많아서 is 많다 + -아서.

The grammar -아서/어서 connects two clauses and often means:

• because
• so
• and so

So here it gives the reason for the next event:

• 모기가 많아서 = because there were a lot of mosquitoes

Also note the form:

• dictionary form: 많다
• connected form: 많아서

It is not 많다서.

Because Korean usually omits possessives like my when the owner is obvious, especially with body parts.

So 다리를 물렸어요 naturally means:

• I got my leg(s) bitten
• I got bitten on the leg(s)

Since the speaker is the one who got bitten, Korean does not need to say 내 unless the speaker wants extra emphasis.

In Korean, body parts are often treated as the direct object with verbs like 물다 and 물리다.

So Korean thinks of it more like:

• bite the leg
• get the leg bitten

That is why 다리를 is natural here.

English often uses on the leg, but Korean commonly uses the object marker instead.

Because 물었어요 means bit from the biter’s point of view, while 물렸어요 means got bitten from the victim’s point of view.

Compare:

• 모기가 다리를 물었어요 = the mosquito bit my leg
• 다리를 물렸어요 = I got my leg bitten / I got bitten on the leg

In this sentence, the speaker is talking about what happened to them, so 물렸어요 is the natural choice.

두 번 simply means twice.

두 번이나 adds emphasis, something like:

• as many as twice
• no less than twice
• twice, of all things

It often shows surprise, annoyance, or that the number feels bigger than expected.

So here 두 번이나 물렸어요 sounds like the speaker is emphasizing how unpleasant it was.

Because Korean often drops the subject when it is clear from context.

Here, once you hear 물렸어요, it is natural to understand that the speaker is talking about themselves:

• I got bitten

Korean does this very often. Sentences sound more natural when obvious pronouns are omitted.

Not necessarily. In Korean, when clauses are connected with -아서/어서, the time can often be understood from the overall context and especially from the final verb.

So:

• 모기가 많아서 다리를 두 번이나 물렸어요

is a completely natural way to say that, because there were many mosquitoes, the speaker got bitten.

The past meaning is clear from 물렸어요 and the situation as a whole.

Because it is already obvious what did the biting.

The first clause introduces mosquitoes:

• 공원에 모기가 많아서

So in the second clause, Korean can simply say:

• 다리를 두 번이나 물렸어요

and the listener naturally understands by mosquitoes.

You could add 모기한테 for extra clarity, but it would sound more repetitive:

• 공원에 모기가 많아서 모기한테 다리를 두 번이나 물렸어요

The original version is smoother and more natural.