Kad se popnemo na brdo, vidimo rijeku i most.

Yes. Kad and kada both mean when.

• kad is very common in everyday speech
• kada can sound a bit more formal or a bit more emphatic

So Kad se popnemo na brdo... and Kada se popnemo na brdo... are both correct.

Croatian often leaves out subject pronouns because the verb ending already shows who is doing the action.

• popnemo = we climb up / we get up
• vidimo = we see

So mi is not necessary. You can add mi only if you want emphasis or contrast, for example: Kad se mi popnemo na brdo...

Because the verb is popeti se, not just popeti.

In other words, se is part of the verb here. It does not literally mean ourselves in English. Learners often try to translate it word-for-word, but it is better to think of popeti se as one unit meaning to climb up / to get up onto.

So:

• popeti se = to climb up
• se popnemo = we climb up / we get up

Popnemo is the 1st person plural present form of popeti se.

So the parts are:

• po- = prefix
• -pnemo = the verb form meaning we
• full form: popnemo se in dictionary order would relate to popeti se

In actual sentence order, the clitic se comes before the verb: se popnemo.

This is about aspect, which is very important in Croatian.

• popeti se is perfective
• penjati se is imperfective

Here, popnemo se suggests a completed climb: we get up the hill, and then we see the river and bridge.

So the feeling is:

• Kad se popnemo na brdo... = when we get up the hill / once we’ve climbed up
• Kad se penjemo na brdo... = when/while we are climbing the hill

The sentence uses perfective because the seeing happens after reaching the top or after completing the upward movement.

Because Croatian uses different cases depending on whether there is movement toward a place or location at a place.

With na:

• na + accusative = motion toward/onto
• na + locative = location on/at

So:

• popnemo se na brdo = we climb up onto/up the hill
• na brdu = on the hill

Here there is movement, so brdo is in the accusative singular, which happens to look the same as the nominative for this neuter noun.

Because it is the direct object of vidimo.

The verb vidjeti takes an object in the accusative case.
So:

• nominative: rijeka
• accusative: rijeku

That is why the sentence has vidimo rijeku.

It is also a direct object, so it is also in the accusative. But with many masculine inanimate nouns, the accusative singular looks the same as the nominative singular.

So:

• nominative: most
• accusative: most

That is completely normal.

A useful contrast:

• vidim most = I see a bridge
• vidim čovjeka = I see a man

The second one changes because čovjek is masculine animate.

Yes. Croatian often uses the present tense in this kind of sentence, especially in clauses with kad.

This can sound:

• general/habitual: whenever we climb up the hill, we see the river and the bridge
• narrative/immediate: when we get up the hill, we see the river and bridge

If you want to make a one-time future event especially explicit, many speakers would say:

• Kad se popnemo na brdo, vidjet ćemo rijeku i most.

So the original sentence is correct, but its most natural reading is often a general or repeated situation.

No, Kad popnemo se na brdo is not natural Croatian.

Se is a clitic, and clitics usually go near the beginning of the clause, in a fixed position. That is why Croatian says:

• Kad se popnemo na brdo...

not:

• Kad popnemo se na brdo...

This is one of the word-order rules learners need to get used to: Croatian word order is flexible in some ways, but clitics such as se are much less free.

Fairly flexible, but not completely free.

The original order is neutral:

• Kad se popnemo na brdo, vidimo rijeku i most.

You can change the order for emphasis, for example:

• Rijeku i most vidimo kad se popnemo na brdo.

That puts more focus on rijeku i most.

But even when you move things around, se still has to obey clitic placement rules.

Because Croatian has no articles like a or the.

Whether something is definite or indefinite is usually understood from context.

So brdo, rijeku, and most can mean:

• a hill / the hill
• a river / the river
• a bridge / the bridge

The exact English translation depends on the situation.

Because vidjeti and gledati are different verbs.

• vidjeti = to see
• gledati = to look at / to watch

Here the idea is that, once we are up on the hill, the river and bridge come into view. So vidimo is the natural choice.

If you said gledamo rijeku i most, it would mean something more like we are looking at the river and the bridge.