bi oneun nareneun bajiga jeotgi swiwoseo yoksireseo baro mallyeoyo.

Both are possible. 비 오는 날 is a very common, natural shortened form meaning a day when it rains / a rainy day.

In Korean, when a clause directly describes a noun, the subject marker is often omitted if the meaning is clear. So:

• 비 오는 날 = rainy day
• 비가 오는 날 = a day when rain comes / a day when it rains

The version without 가 sounds a little lighter and more natural in many everyday contexts.

날에 means on a day or on that day. Adding 는 makes it 날에는, which gives a topic or contrast feeling.

So 비 오는 날에는 means something like:

• on rainy days
• when it’s a rainy day
• as for rainy days

The 는 often suggests contrast, like on rainy days (as opposed to other days).

Here, 바지 is the thing being described as likely to get wet, so it takes the subject marker 가.

The structure is:

• 바지가 = the pants
• 젖기 쉽다 = are easy to get wet / tend to get wet easily

So 바지가 젖기 쉬워서 means because the pants get wet easily or because the pants are likely to get wet.

This is a very useful pattern:

• 젖다 = to get wet
• 젖기 = getting wet / to get wet
• 쉽다 = to be easy
• 쉬워서 = because it is easy / because it tends to happen easily

So 젖기 쉽다 literally means to be easy to get wet, but in natural English it often means to get wet easily or to be prone to getting wet.

Then -아서/어서 in 쉬워서 gives the reason: because it gets wet easily.

They are similar, but not exactly the same.

• 잘 젖다 = gets wet easily / gets wet a lot
• 젖기 쉽다 = is prone to getting wet / is liable to get wet

잘 젖다 sounds a bit more direct and conversational.
젖기 쉽다 sounds slightly more descriptive or explanatory.

In this sentence, 젖기 쉬워서 works well because it naturally leads into the reason for drying the pants right away.

Use 에서 because the action 말리다 is happening in the bathroom.

A quick comparison:

• 에 = to, at, in, for location/existence/destination
• 에서 = at/in a place where an action happens

So:

• 욕실에 있어요 = it is in the bathroom
• 욕실에서 말려요 = I dry it in the bathroom

Since drying is an action, 에서 is correct.

바로 here means right away, immediately, or directly.

So 욕실에서 바로 말려요 means something like:

• I dry them right away in the bathroom
• I immediately dry them in the bathroom

Depending on context, 바로 can sometimes also feel like right there, but here the most natural meaning is immediately.

It is from 말리다, which means to dry something or to hang something up to dry.

Compare:

• 마르다 = to dry, to become dry
• 말리다 = to dry something

So:

• 바지가 말라요 = the pants dry / become dry
• 바지를 말려요 = I dry the pants

In this sentence, the speaker is doing the action to the pants, so 말려요 is the right form.

Yes. Korean often omits subjects when they are obvious from context.

In 욕실에서 바로 말려요, the understood subject is something like I or we, depending on the situation. English usually needs that subject, but Korean does not.

So the sentence naturally implies: On rainy days, pants get wet easily, so I dry them right away in the bathroom.

Here it expresses reason/cause: because, so.

So the structure is:

• 바지가 젖기 쉬워서 = because the pants get wet easily
• 욕실에서 바로 말려요 = I dry them right away in the bathroom

In other words: Because pants tend to get wet on rainy days, I dry them right away in the bathroom.

In Korean, nouns usually do not change form for singular vs. plural the way English does.

So 바지 can mean:

• pants
• a pair of pants
• trousers in general

Korean does have plural markers like 들, but they are not used as often as English plurals. So 바지가 here is perfectly normal even though the natural English translation uses pants.

말려요 is in the 해요체 style, which is the standard polite conversational form.

That makes the whole sentence sound polite but natural for everyday speech.
The dictionary form would be 말리다, and the plain present polite form is 말려요.

So this is a good sentence pattern for normal conversation.