mikkeureoun naerimageseo neomu apseogaji ma.

Because 미끄럽다 is being used to describe a noun.

In Korean, when a descriptive verb/adjective modifies a noun, it changes form:

• 미끄럽다 = to be slippery
• 미끄러운 내리막 = a slippery downhill slope

This is the noun-modifying form.
Also, 미끄럽다 is a ㅂ-irregular adjective, so it becomes 미끄러운, not 미끄럽은.

Compare:

• 예쁘다 → 예쁜
• 어렵다 → 어려운
• 미끄럽다 → 미끄러운

내리막 means a downhill slope, descent, or downhill section.

It refers to the part of a road, hill, path, or slope that goes downward. Depending on context, it could be:

• a road
• a hiking trail
• a ski slope
• any sloping surface

You may also see 내리막길, which is very similar.

• 내리막 = downhill / descent
• 내리막길 = downhill road/path

In many situations, they are interchangeable.

Here, 에서 marks the place where the action happens.

So 내리막에서 means:

• on the downhill slope
• while on the downhill section

Korean often uses:

• 에 for destination, existence, or time
• 에서 for the location where an action takes place

Since 앞서가다 is an action, 에서 sounds natural here.

So the idea is:

• 내리막에서 = on the downhill, in that area/situation
• not so much to the downhill, but while being there

앞서가다 literally means to go ahead, to move ahead of others, or to get out in front.

In this sentence, it most likely means:

• don’t move too far ahead of the other person/people
• don’t get too far in front

It does not mainly mean go faster downhill.
It is more about position relative to others.

So if two people are walking, hiking, skiing, or biking, this would mean:

• Don’t go too far ahead of me/us.

They are similar, but not identical.

• 먼저 가다 = to go first
• 앞서가다 = to go ahead / move out in front

먼저 가다 focuses on order: someone goes before someone else.
앞서가다 focuses more on being physically ahead.

So in this sentence, 앞서가다 fits well because the warning is about being too far in front on a slippery downhill slope.

Here it means too or excessively.

너무 can sometimes mean very in casual Korean, but in a warning like this, it clearly means:

• too much
• more than is safe or appropriate

So 너무 앞서가지 마 means:

• don’t go too far ahead
• don’t get too much in front

-지 마 is the pattern used to say don’t do ...

It attaches to the verb stem:

• 가다 → 가지 마
• 먹다 → 먹지 마
• 앞서가다 → 앞서가지 마

So:

• 앞서가지 마 = don’t go ahead

This form is casual/plain, so it is used with:

• friends
• younger people
• children
• someone you speak informally with

More polite versions would be:

• 앞서가지 마세요
• 미끄러운 내리막에서 너무 앞서가지 마세요.

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

In a command, the understood subject is usually you.
So even without 너는, the meaning is naturally:

• (You) don’t go too far ahead.

Including 너는 is possible, but it would usually add emphasis or contrast:

• 너는 너무 앞서가지 마.
  = You, don’t go too far ahead.

Most of the time, Korean sounds more natural without the subject if it is already clear.

Not exactly.

It is telling them not to get too far ahead. That could happen because they are moving fast, but the main idea is being ahead of others, not just speed itself.

If you wanted to say Don’t go too fast, Korean would more naturally use something like:

• 너무 빨리 가지 마.
• 속도를 너무 내지 마.

So:

• 앞서가지 마 = don’t get ahead
• 빨리 가지 마 = don’t go fast

This is casual speech.

The ending -마 without 요 is informal, so it sounds like something said to:

• a friend
• a younger sibling
• a child
• someone close to you

More polite versions:

• 미끄러운 내리막에서 너무 앞서가지 마세요.
• 미끄러운 내리막에서 너무 앞서가지 마십시오.
  (more formal, less common in everyday conversation)

Because 앞서가다 is a single verb, while -지 마 comes from 말다 and is written separately in standard spacing.

So:

• 앞서가다 = one dictionary verb
• 앞서가 + 지 마 = verb stem + don’t

That is why the sentence is written:

• 앞서가지 마

and not:

• 앞서 가다 for the dictionary verb
• or 가지마 in standard writing

So the spacing in the sentence is correct.