jeo bajineun danchuga manhaseo ibeul ttae sigani geollyeoyo.

는 marks 저 바지 as the topic: as for those pants...

Here, 저 바지는 sets up what the sentence is talking about, and the rest of the sentence explains something about them. Using 는 can also give a slight contrastive feeling, like:

• As for those pants, they take time to put on.

That contrast can imply they are different from other clothes.

In Korean, 바지 normally refers to pants/trousers as a single clothing item, even though English uses a plural form.

So:

• 저 바지 = those pants / those trousers

You usually do not need 들 here. Adding 들 would sound unnatural in most normal situations unless you really mean multiple pairs of pants.

In 단추가 많다, the thing that is numerous gets 가/이.

So:

• 단추가 많다 = there are many buttons / the buttons are many

This is the normal pattern with 많다.
You would not usually say 단추를 많다 because 많다 does not take a direct object marked with 를/을.

단추는 많아서 could be used in a special contrastive context, but 단추가 많아서 is the neutral, natural form.

Here -아서/어서 means because or so.

• 단추가 많아서 = because there are many buttons
• more literally: having many buttons, ...

It connects the reason to the result:

• 단추가 많아서 입을 때 시간이 걸려요
• Because there are many buttons, it takes time when putting them on.

Both are possible, but they feel slightly different.

• 입을 때 = when putting them on
• 입을 때는 = when it comes to putting them on / when putting them on, specifically

The version without 는 is more neutral and straightforward. Adding 는 would add contrast or emphasis, as if comparing that time with some other time, such as taking them off.

For example:

• 입을 때는 시간이 걸리지만 벗을 때는 빨라요.
• When putting them on, it takes time, but taking them off is quick.

입다 can mean both to wear and to put on, depending on context.

In this sentence, 입을 때 clearly means when putting on the pants, because the sentence talks about the process taking time due to many buttons.

So:

• 바지를 입다 can mean:
  • to wear pants
  • to put on pants

Context tells you which meaning is intended.

Literally, it means time takes/catches in Korean structure, but naturally it means:

• it takes time
• it takes a while

The pattern is:

• 시간이 걸리다 = to take time

Examples:

• 오래 걸려요. = It takes a long time.
• 10분 걸려요. = It takes 10 minutes.

So in this sentence:

• 입을 때 시간이 걸려요 = it takes time when putting them on

Korean often leaves out subjects when they are obvious from context.

In English, we might say:

• It takes time to put them on.

But Korean does not need a dummy subject like it. The idea is understood from context:

• 저 바지는 ... 시간이 걸려요
• As for those pants, ... it takes time.

The sentence is completely natural without adding an extra subject.

저 means something like that over there or those over there, depending on the noun.

Because 바지 means pants in English, it is naturally translated as those pants.

Korean itself is not marking English-style singular/plural here. The important point is that 저 indicates something away from both the speaker and the listener.

Quick comparison:

• 이 = this, near the speaker
• 그 = that, near the listener or previously mentioned
• 저 = that over there, away from both

Because 있다 and 많다 work differently.

The natural Korean idea is:

• 단추가 많다 = there are many buttons / the buttons are numerous

You do not say 단추를 많이 있다. That is ungrammatical.

If you used 있다, a more natural structure would be:

• 저 바지에는 단추가 많이 있어요.
• There are many buttons on those pants.

But in your sentence, 단추가 많아서 is simpler and more natural.

Both can express a reason, but they feel a little different.

• 많아서 = because there are many buttons / so there are many buttons
• 많으니까 = since there are many buttons / because there are many buttons

In this sentence, 많아서 sounds smooth and descriptive.
많으니까 can sound a bit more direct, explanatory, or speaker-judgment-based.

Both are possible:

• 단추가 많아서 입을 때 시간이 걸려요.
• 단추가 많으니까 입을 때 시간이 걸려요.

The first one is slightly more neutral and natural in plain description.

Usually, 시간이 걸리다 is the standard expression for to take time.

So this is natural:

• 시간이 걸려요

Using 시간이 들어요 is much less common for this meaning and would not be the usual choice here.

For time, remember:

• 시간이 걸리다 = to take time

For money or effort, Korean may also use things like:

• 돈이 들다 = money is required / it costs money
• 노력이 들다 = effort is required

So in this sentence, 걸려요 is the correct natural choice.

Yes. Even if the meaning is already clear, this Korean sentence can map to several natural English versions, such as:

• Those pants have a lot of buttons, so they take a while to put on.
• Because those pants have so many buttons, it takes time to put them on.
• Those pants take time to put on because they have a lot of buttons.

This happens because Korean and English organize information differently. Korean often presents the topic first, then the reason, then the result.