Wij lopen de berg op.

Because Dutch often uses a postposition with movement expressions. In English, we say up the mountain. In Dutch, that idea is commonly expressed as de berg op — literally the mountain up.

So in Wij lopen de berg op, the words de berg op belong together and express the direction of movement.

It means up the mountain or up the hill.

A useful way to see it is:

• de berg = the mountain / hill
• op = up / onto

But in Dutch, the natural order here is de berg op, not op de berg, when the meaning is movement upward.

Because op de berg usually describes location, not direction.

• Wij lopen op de berg = we are walking on the mountain
• Wij lopen de berg op = we are walking up the mountain

So the sentence you have is about going upward, not just being on the mountain.

Because naar de berg means toward the mountain or to the mountain, not up the mountain.

Compare:

• Wij lopen naar de berg = we walk toward the mountain
• Wij lopen de berg op = we walk up the mountain

So naar gives the destination or direction toward something, while de berg op specifically shows upward movement along it.

Not really, at least not in the usual sense.

The verb lopen is normally intransitive: it means to walk, and it usually does not take a direct object. Here, de berg op works as a directional phrase. It may look a bit object-like to an English speaker because there is no preposition before de berg, but grammatically the whole chunk de berg op expresses direction: up the mountain.

Yes, very closely.

A learner will often find it helpful to connect this sentence with oplopen = to walk up.

In a main clause, Dutch often splits this kind of form:

• Wij lopen de berg op

When the verb stays together, you often see:

• de berg oplopen
• omdat wij de berg oplopen
• wij willen de berg oplopen

So treating it as related to separable-verb behavior is useful, even though grammars may describe de berg op more specifically as a directional construction.

Yes. We lopen de berg op is very common and natural, especially in speech.

• wij = fuller form
• we = unstressed, very common everyday form

Wij can sound a bit more emphatic, careful, or contrastive in some contexts, but both are correct.

It is the present tense, plural form.

Dutch present tense for lopen goes like this:

• ik loop = I walk
• jij loopt = you walk
• hij/zij loopt = he/she walks
• wij lopen = we walk
• jullie lopen = you walk
• zij lopen = they walk

So Wij lopen de berg op is simply We walk / are walking up the mountain.

Then you will often see oplopen together at the end.

For example:

• omdat wij de berg oplopen = because we are walking up the mountain
• wij willen de berg oplopen = we want to walk up the mountain

This is a very useful pattern to learn, because it helps you connect lopen ... op in the main clause with oplopen when the verb comes later.

Not always. Depending on context, berg can mean mountain or hill.

So Wij lopen de berg op could be translated as:

• We are walking up the mountain
• We are walking up the hill

The exact choice depends on what kind of berg the speaker is talking about.