Learn Hardcore Korean: saneseoneun neomu apseogaji ma. - Do not go too far ahead in the mountains.

Breakdown of saneseoneun neomu apseogaji ma.

Why is it 산에서는 instead of just 산에서?

산에서 simply means in the mountains or at the mountain(s).

Adding 는 makes it 산에서는, which gives the phrase a topic/contrastive feeling. It is like saying:

So 산에서는 너무 앞서가지 마 sounds a little more like a general warning for that situation: In the mountains, don’t go too far ahead.

What does 너무 mean here?

Here 너무 means too much or excessively, not just very.

So it is not simply saying don’t go ahead. It is saying:

That is a very common use of 너무 before something negative or undesirable.

What does 앞서가다 mean in this sentence?

앞서가다 literally contains the idea of going ahead/in front.

In this sentence, it means something like:

In a mountain/hiking context, it usually means physically going farther ahead than the other people you are with.