There’s usually a whole theory in textbooks about various types of so-called conditional sentences – sentences like if I..., I’d... (add your ideas here).
Actually, it’s quite simple. There are two types of such sentences: the first one is what could be (or could had been) but it’s not (e.g. if I were rich, I’d...). So it’s a kind of missed opportunity, a wish that’s not fulfilled, and you say why.
In such sentences in Croatian, what was not, but could be, is expressed in conditional, and the reason (or obstacle) is expressed simply in the past tense or the present tense, starting with the magic word da. Such clauses behave like normal clauses, perf. verbs cannot be used in the present tense. They simply refer to imagined actions or conditions that were necessary for the other part (in conditional) to happen. For example:
Da je bilo toplo, otišli bismo na plažu. If it had been warm, we would have gone to the beach. (past)
Da je toplo, otišli bismo na plažu. If it were warm, we would go to the beach. (present)
The Croatian sentences are simpler than English, since there’s no special usage of tenses. If some obstacle holds now, it’s expressed in the present tense. If it happened in the past, the obstacle is in the past tense. The other part is in conditional.
There’s an exception to this pattern: if you express what could have happened with the verb moći (...) can + another verb in infinitive, you should use just the past tense instead of the conditional; it works for the past only:
Da je bilo toplo, mogli smo otići na plažu. If it had been warm, we could have gone to the beach. (past)
English has also mixed conditional sentences, where the condition and unrealized outcome are in different tenses, e.g. if we had won the lottery, we would be rich. Since Croatian unrealized outcomes are in the conditional form, which doesn’t show tense, Croatian sentences really don’t tell when the outcome could have happened. You can express a different moment with adverbs of time, e.g. the present moment with sad(a) now:
Da smo osvojili lutriju, sad bismo bili bogati. If we had won the lottery, we would be rich. (past-present)
Another way to express ‘present’ (rather, present-indefinite future) unrealized outcomes is with kad(a). Now both parts are in conditional:
Kad bih imao puno novaca, kupio bih skup auto. If I had a lot of money, I’d buy an expensive car. (present)
Finally, recall that in colloquial speech and writing, conditional is usually expressed with just bi for all persons: you will mostly hear sad bi bili bogati instead of the formal sentence above, and so on.
That much about such sentences.
The second type of conditional sentences is completely different. Now we simply say what is going to happen if something gets fulfilled.
In English, such sentences look like if I get rich, I’ll... (add more ideas). They are similar to the first type since both use the word if. Not so in Croatian: first, another word is used – ako:
Ako je toplo, idemo na plažu. If it’s warm, we go to the beach. (now, near future)
Ako bude toplo, ići ćemo na plažu. If it’s warm, we’ll go to the beach. (future)
As you see, the structure is completely different now: the action is expressed in the present or future tense, and the part after ako uses the same rules as near future or future sentences that use kad: bude instead of usual forms of the verb biti, the potential future tense (again not strictly respected in speech), etc.:
Ako budemo igrali loše, sigurno ćemo izgubiti. If we play poorly, we’ll lose for sure. (future)
Actually, if you search the Internet, ako ćemo... is more common than ako budemo... (or ako budem vs. ako ću, etc.). Try it yourself. Still, use of the common future tense after ako and kad is considered a mistake in formal writing.
In literature, you will sometimes find that in such clauses, instead of ako + forms of (bude), verb fronting + li² is used:
Bude li toplo, ići ćemo na plažu.
Budemo li igrali loše, sigurno ćemo izgubiti.
This is extremely rare in speech. There’s no difference in meaning; it’s mostly used when the verb (bude) is used.
Of course, the conjunction ako holds the first position in the clause, and all second-position words come right after it.
As you can see, ako-clauses are very similar to kad-clauses, but there’s more freedom in ako-clauses: since the condition (if it’s warm) and the action (we’ll go to the beach) are not necessarily close in time, the condition can be expressed in the past tense, and the action in any tense:
Ako se vratila, nazvat će me. If she has came back, she’ll call me. (past, future)
Of course, it’s possible to use imperative or even conditional in the main (action) part – everything, except the past tense:
Ako se vratila, odmah je nazovi. If she has came back, call her immediately. (past, imperative)
There’s another type of sentences, not frequently discussed, kind of opposite of what I have just described. The reasons in the conditional sentences were important – they are precisely why something was not done or isn’t done. However, it’s easy to turn such sentences upside down: something happened (or didn’t) regardless of a condition. This is what I mean:
Even if it had been cold, we would have gone to the beach.
It wasn’t cold, but even if it were, the second part would happen, regardless of the condition. In Croatian, surprisingly, you just add an i¨ before da and the whole sentence changes its meaning completely:
I da je bilo hladno, otišli bismo na plažu. Even if it had been cold, we would have gone to the beach.
As usual, the word i¨ is ‘glued’ and it’s not counted: all second-position words come after the da.
Adding another word – čak – before i da further emphasizes irrelevance of the condition. Such expression really translates as even if; only i da is more it doesn’t matter:
Čak i da je bilo hladno, otišli bismo na plažu. (emphasis)
Another option is that it was cold, but we still did it. The grammar is then of the second type (ako....). Then you would add i¨ to ako, but then they get fused to a single word iako:
Iako je bilo hladno, otišli smo na plažu. Although it was cold, we went to the beach.
The combined word iako is used like i da – only in the past and present tenses. To express that you will do something in the future, regardless of something else, use separate i ako:
I ako bude hladno, otići ćemo na plažu. Even if it’s cold, we’ll go to the beach.
Finally, it’s possible to ask hypothetical questions, e.g. what would..., if.... Everything said before about the first type of sentences (using da) still applies:
Gdje bismo išli, da je bilo toplo? Where would we have gone, if it had been warm? (past)
Što bi radio, da pada kiša? What would you do, if it were raining? (present)
Of course, it’s possible to ask about future, using ako:
Hoćeš li je nazvati, ako se vrati? Will you call her, if she comes back?
This applies to all kinds and ways of making questions.
Clauses can be reordered, you can start questions with da or ako, but you still need to separate them by a comma.