saneseoneun kkok deungsanhwareul sineo.

산에서는 breaks down like this:

• 산 = mountain
• 에서 = at/in, marking the place where an action happens
• 는 = topic/contrast marker

So 산에서는 means something like:

• in the mountains
• when you’re in the mountains
• as for in the mountains

You would not use 산에 here because 에 is more often used for destination or location with certain verbs, while 신다 is an action happening in a place, so 에서 fits better.

This is a very common question. 에서는 is really 에서 + 는.

• 에서 tells you the action happens there.
• 는 adds a topic or contrast feeling.

So 산에서 꼭 등산화를 신어 would simply mean Wear hiking boots in the mountains.

But 산에서는 꼭 등산화를 신어 adds a nuance like:

• In the mountains, make sure you wear hiking boots
• At least when you’re in the mountains, wear hiking boots
• When it comes to the mountains, hiking boots are a must

The 는 often makes it sound a little more like advice, a rule, or something being contrasted with other places.

No. Here 꼭 does not mean only.

In this sentence, 꼭 means:

• definitely
• certainly
• be sure to
• without fail

So the sentence has the feeling of Make sure you wear hiking boots in the mountains.

If you wanted to say only, Korean usually uses 만, not 꼭.

를 is the direct object marker.

• 등산화 = hiking shoes / hiking boots
• 등산화를 = hiking shoes/boots + object marker

The verb 신다 means to wear footwear, so the thing being worn is the object of the verb. That is why 를 is used.

So the structure is:

• 등산화를 신어 = wear hiking boots

Korean uses different verbs for wearing different kinds of items.

• 입다 = wear clothes
• 신다 = wear shoes, boots, socks
• 쓰다 = wear a hat / glasses
• 끼다 = wear rings, gloves, etc.

Since 등산화 means hiking shoes/boots, the correct verb is 신다.

So:

• 옷을 입다 = wear clothes
• 모자를 쓰다 = wear a hat
• 등산화를 신다 = wear hiking boots

This is very different from English, where wear works for almost everything.

In this sentence, it is most naturally understood as a casual command/advice:

• Wear hiking boots
• Be sure to wear hiking boots

The ending -어 is informal and casual. Depending on context, forms like this can sometimes look like plain statements, but here, because of 꼭 and the overall situation, it strongly sounds like advice or instruction.

So a good way to understand it is:

• 산에서는 꼭 등산화를 신어.
  In the mountains, make sure you wear hiking boots.

Yes, most likely the subject is an understood you.

Korean often leaves out subjects when they are obvious from context. In advice, instructions, and commands, you is very often omitted.

So this sentence does not need to say:

• 너는 산에서는 꼭 등산화를 신어.

That would be possible in some contexts, but usually it is unnecessary.

It can also sound a bit more general, like:

• People should wear hiking boots in the mountains.

But in everyday conversation, it most often feels like advice directed at you.

신어 is casual / intimate speech. You would use it with:

• close friends
• younger people
• children
• someone you speak casually with

If you want to say it politely, you could say:

• 산에서는 꼭 등산화를 신으세요. = Please make sure to wear hiking boots in the mountains.
• 산에서는 꼭 등산화를 신어야 해요. = You really should wear hiking boots in the mountains.

So the original sentence is not rude by itself, but it is informal.

In Korean, nouns often do not have to show singular vs. plural clearly unless it matters.

So 등산화 could be understood as:

• hiking shoes
• hiking boots
• a pair of hiking boots, depending on context

English usually forces you to choose singular or plural, but Korean often does not. That is why translations may vary slightly even when the Korean sentence stays the same.

Yes, that would also be correct.

The difference is mainly nuance:

• 산에서 꼭 등산화를 신어 = straightforward advice: Wear hiking boots in the mountains
• 산에서는 꼭 등산화를 신어 = more topical/contrastive: When it comes to the mountains, make sure you wear hiking boots

So the original version with 는 feels a little more like a general rule or emphasized advice.