biga waseo naerimageun aju mikkeureowo.

Here, 와서 means because it’s raining.

It comes from the verb 오다 (to come), which is also the verb used for rain:

• 비가 오다 = to rain

Then Korean adds -아서/어서 to show a reason or cause:

• 비가 와서 = because it’s raining / since it’s raining

This ending can also sometimes mean and then, but in this sentence it clearly gives a reason:

• Because it’s raining, the downhill is very slippery.

Because 비 is the subject of the verb 오다 in this expression.

In Korean, rain is expressed as:

• 비가 오다 = rain comes → it rains

So:

• 비가 = the rain as the subject
• 오다 = to come (used idiomatically for weather like rain)

You do not say 비를 오다 because 비 is not the object here.

This is a contraction.

The verb is:

• 오다 = to come

Its stem is:

• 오-

When you attach -아서, you would expect:

• 오아서

But Korean contracts this naturally to:

• 와서

So:

• 오다 → 와서
• 오다 → 와요
• 오다 → 왔어요

This is a very common pattern and something learners just need to get used to.

내리막 means a downhill slope, a downward stretch, or a slope going down.

It often refers to:

• a downhill road
• a sloping path
• a downward section of terrain

A related word you may also see is:

• 내리막길 = downhill road / downhill path

In many situations, 내리막 by itself sounds natural and understandable.

The particle 은 marks 내리막 as the topic.

So 내리막은 has a feeling like:

• as for downhill sections
• downhills are...
• the downhill, though,...

This makes sense because the speaker is talking about downhill areas in general under the current conditions.

Compare:

• 내리막은 아주 미끄러워 = As for downhill sections, they’re very slippery
• 내리막이 아주 미끄러워 = more like The downhill is very slippery with stronger subject focus

Using 은 often feels more natural when giving a general comment or warning.

Because 미끄럽다 is a ㅂ-irregular adjective.

The dictionary form is:

• 미끄럽다 = to be slippery

When you conjugate it before a vowel, the ㅂ changes to 우/오-type sound:

• 미끄럽 + 어요 → 미끄러워요
• 미끄럽 + 어 → 미끄러워

So:

• 미끄러워 = casual form
• 미끄러워요 = polite form

This same pattern appears in words like:

• 덥다 → 더워요 (to be hot)
• 어렵다 → 어려워요 (to be difficult)

In Korean grammar, 미끄럽다 is usually treated as a descriptive verb (often called an adjective in learner-friendly explanations).

In English, we think of it as an adjective:

• slippery

But in Korean, words like 크다 (to be big), 좋다 (to be good), and 미끄럽다 (to be slippery) behave like verbs in conjugation.

So 미끄러워 is the conjugated present-tense form meaning:

• is slippery

Korean often leaves out things that are understood from context.

In English, you might say:

• Because it’s raining, the downhill road is very slippery.

In Korean, if the context already makes it clear what is being talked about, you can simply say:

• 내리막은 아주 미끄러워

This can imply:

• downhill roads
• downhill areas
• the slope
• the downward stretch

Korean does this kind of omission all the time, and it sounds natural.

아주 means very.

So:

• 아주 미끄러워 = very slippery

It emphasizes the degree of slipperiness.

Some similar words are:

• 매우 = very (more formal/written)
• 너무 = very / too (common in speech, but can sometimes sound like too)

In this sentence, 아주 sounds natural and straightforward.

Grammatically, -아서/어서 can sometimes connect two actions in a sequence, like and then. But in this sentence, the natural meaning is because.

Why? Because the relationship is clearly cause-and-effect:

• it rains
• the downhill becomes slippery

So the listener understands:

• Because it’s raining, the downhill is very slippery

If it were a simple sequence, the meaning would be less natural here.

This sentence is in casual, non-polite speech.

The ending:

• 미끄러워

is informal, so it would be used with:

• friends
• family
• someone younger
• casual narration or inner thoughts

A polite version would be:

• 비가 와서 내리막은 아주 미끄러워요.

A more formal/plain written style could be:

• 비가 와서 내리막은 아주 미끄럽다.

So the original sentence sounds conversational and informal.

Yes, absolutely.

• 내리막은 아주 미끄러워
• 내리막길은 아주 미끄러워

Both are natural.

The difference is that 내리막길 is a bit more explicit because it clearly means a downhill road/path.
내리막 is slightly broader and can refer more generally to a downhill slope or downward section.

So if you want to be extra clear that you mean a road or path, 내리막길 is a good choice.

You can understand the structure that way, but it is not the best natural English translation.

A more literal breakdown is:

• 비가 = rain + subject marker
• 와서 = comes-and / because comes
• 내리막은 = as for the downhill
• 아주 = very
• 미끄러워 = is slippery

But natural English should be:

• Because it’s raining, the downhill is very slippery. or
• The downhill is very slippery because it’s raining.

So a literal gloss can help you study the grammar, but it should not be your final English translation.