Kad završi kiša, otići ćemo na livadu i možda opet vidjeti dugu iznad brda.

Both mean when. Kad is the shorter, very common everyday form, while kada is a bit fuller and can sound slightly more formal or emphatic. In this sentence, kad is completely natural.

After kad in time clauses, Croatian usually does not use the future tense the way English does. Instead, it often uses the present tense of a perfective verb to talk about a future completed action.

So:

• Kad završi kiša = When the rain stops / when the rain is over

Here završi is present tense, but because završiti is a perfective verb, it refers to a completed event in the future.

This is very normal in Croatian:

• Kad dođe, nazvat ću te. = When he/she comes, I’ll call you.

Završi is the 3rd person singular present form of the perfective verb završiti.

Why 3rd person singular? Because the subject is kiša (rain), which is singular:

• kiša završava / završi = the rain ends / stops

In this sentence, the meaning is future because of the time clause with kad.

Kiša is the subject of završi, so it is in the nominative singular.

Base form:

• kiša = rain

Since it is the thing that is doing the action of ending/stopping, nominative is the expected case.

Yes. Both are possible.

Croatian word order is more flexible than English word order. So both:

• Kad završi kiša
• Kad kiša završi

are understandable and natural.

The version in your sentence puts the verb earlier, which can sound slightly more dynamic or flow better stylistically, but there is no major change in meaning.

Because ćemo is a clitic (a short unstressed word), and clitics normally do not stand at the very beginning of a clause.

So if there is no other suitable word before ćemo, Croatian places the infinitive first:

• otići ćemo = we will go

If you had a word before it, then ćemo could follow that word:

• Mi ćemo otići. = We will go.
• Sutra ćemo otići. = We will go tomorrow.

So otići ćemo is the natural choice here.

Because na livadu shows movement toward a place, and that takes the accusative.

• otići na livadu = to go to the meadow

By contrast, na livadi would describe location:

• biti na livadi = to be in/on the meadow

So:

• na livadu = motion, destination
• na livadi = location

This is a very common Croatian pattern with prepositions like na and u.

Because Croatian can omit the repeated future auxiliary when two verbs share the same subject and belong to the same future situation.

So:

• otići ćemo ... i možda opet vidjeti ...

means roughly:

• we will go ... and maybe see again ...

The future meaning of vidjeti is understood from otići ćemo.

A fuller version could be something like:

• ... i možda ćemo opet vidjeti dugu ...

But the sentence as written is perfectly natural and efficient.

Because duga (rainbow) is the direct object of vidjeti (to see), so it must be in the accusative.

Base form:

• duga = nominative singular

Object form:

• dugu = accusative singular

Since duga is a feminine noun ending in -a, its accusative singular changes to -u:

• Vidjeti dugu = to see a rainbow

Because the preposition iznad (above) takes the genitive case.

So:

• brdo = hill
• brda = genitive singular (of the hill / above the hill)

That is why you get:

• iznad brda = above the hill

This is part of the normal case pattern after iznad.

Opet means again.

So možda opet vidjeti dugu means:

• maybe see a rainbow again

Its position is quite natural here. Croatian adverbs like možda (maybe) and opet (again) are fairly flexible, but this order sounds smooth and idiomatic.

Because Kad završi kiša is an introductory subordinate time clause, and Croatian normally separates it from the main clause with a comma.

So the structure is:

• Kad završi kiša, = subordinate clause
• otići ćemo na livadu... = main clause

This is similar to English punctuation in:

• When the rain stops, we’ll go...