Kad završim ovaj kolegij, upisat ću drugi.

Because Croatian often uses the present tense of a perfective verb after time words like kad to talk about a future action that will be completed before the next action happens.

So:

• kad završim = when I finish
• not when I am finishing

This is very natural in Croatian. English does something similar with when I finish, not when I will finish.

Both kad and kada mean when.

The difference is mostly style:

• kad = shorter, very common in everyday language
• kada = a bit more formal or fuller-sounding

So this sentence could also be:

• Kada završim ovaj kolegij, upisat ću drugi.

Both are correct.

Both are correct.

Croatian future tense can often be formed in two common ways:

• upisat ću
• ću upisati

With verbs whose infinitive ends in -ti, if the infinitive comes before ću, the final -i is dropped:

• upisati
  • ću → upisat ću
• završiti
  • ću → završit ću

So upisat ću drugi and ću upisati drugi both mean the same thing.

Ću is the 1st person singular form of the auxiliary verb htjeti used to make the future tense.

So:

• ću = I will

In this sentence:

• upisat ću = I will enroll / I will take

Other forms are:

• ćeš = you will
• će = he/she/it will
• ćemo = we will
• ćete = you will
• će = they will

Croatian usually leaves out subject pronouns when they are already clear from the verb ending.

Here the verbs already show I:

• završim = I finish
• ću in upisat ću = I will

So ja is unnecessary unless you want emphasis or contrast:

• Kad ja završim ovaj kolegij, upisat ću drugi.

That sounds more emphatic, like when I finish...

It is in the accusative case because it is the direct object of završim.

You finish what? → ovaj kolegij

For masculine inanimate nouns like kolegij, the accusative singular looks the same as the nominative singular:

• nominative: ovaj kolegij
• accusative: ovaj kolegij

So the form does not visibly change here, but the case is still accusative.

The noun is understood and left out because it would be repetitive.

So:

• upisat ću drugi really means upisat ću drugi kolegij

This is very common in Croatian. If the noun is obvious from context, it can be omitted.

Here drugi agrees with the implied noun kolegij.

Literally, drugi can mean second or other/another, depending on context.

In this sentence, it most naturally means:

• another one
• more specifically, another course

Why? Because the speaker is talking about finishing one course and then enrolling in a different one. So English usually translates it as another rather than strictly the second.

Because završiti and upisati are perfective verbs. They describe a completed action as a whole.

That fits this sentence well:

• first, the course is completed
• then, another course is enrolled in

This is a one-time sequence of completed actions, so perfective verbs are the natural choice.

Very roughly:

• završiti = finish, complete
• završavati = be finishing / finish repeatedly / focus on the process
• upisati = enroll in, sign up for
• upisivati = be enrolling, enroll repeatedly, or focus on the process

Because Kad završim ovaj kolegij is a subordinate time clause, and it is separated from the main clause with a comma.

So the structure is:

• Kad završim ovaj kolegij, = subordinate clause
• upisat ću drugi. = main clause

This comma is standard Croatian punctuation.

Yes. Croatian word order is more flexible than English, although not completely free.

For example, you could also say:

• Upisat ću drugi kad završim ovaj kolegij.

That still means the same thing.

The difference is mostly in focus:

• Kad završim ovaj kolegij, upisat ću drugi.
  This highlights the time condition first.
• Upisat ću drugi kad završim ovaj kolegij.
  This highlights the main action first.

But you must still respect normal placement rules for short words like ću.

Usually it is closest to when I finish or once I finish.

In context, it often implies that the second action will happen after the first one is completed, but the Croatian wording itself is naturally just kad završim.

So a good sense is:

• when I finish
• once I finish

rather than a strongly explicit after I have finished, even though the meaning can be very close.