Learn Hardcore Croatian: Isti majstor će sutra provjeriti krov, jer mislimo da malo prokišnjava. - The same repairman will check the roof tomorrow, because we think it is leaking a bit.

isti majstor = the same workman / the same repairman (as the one we already mentioned or both know about).
It does not mean this in the sense of this particular one here (that would be ovaj majstor).
It also isn’t just that (taj majstor) – isti always implies comparison with some earlier reference: the same one as before.

You can also emphasize it more:

onaj isti majstor = that same repairman (you remember which one!)

Why is the word order Isti majstor će sutra provjeriti krov and not Isti majstor sutra će provjeriti krov?

Croatian has a rule that short forms like će (future auxiliary) prefer to stand in second position in the clause, after the first stressed word or phrase.

In this sentence, the first phrase is Isti majstor, so the neutral placement is:

Isti majstor će sutra provjeriti krov.

Putting sutra before će like:

✗ Isti majstor sutra će provjeriti krov sounds unnatural, because će is being pushed further to the right instead of staying near the beginning.

More natural alternatives would be:

Isti majstor će sutra provjeriti krov. (given version)
Sutra će isti majstor provjeriti krov.

Both of those respect the second‑position tendency of će.

Can sutra go at the beginning or the end? For example, are these okay?

Sutra će isti majstor provjeriti krov.
Isti majstor će provjeriti krov sutra.

Yes, both are grammatically fine; the difference is mostly in emphasis:

Sutra će isti majstor provjeriti krov.
Emphasis on tomorrow – Tomorrow, the same repairman will check the roof.
Isti majstor će provjeriti krov sutra.
Slight emphasis on who will do it (the same guy), with sutra just telling you when at the end.

The original:

Isti majstor će sutra provjeriti krov is quite neutral: subject (Isti majstor), then auxiliary će, then time (sutra), then verb + object.

Why is provjeriti (perfective) used here instead of provjeravati?

Croatian verbs have aspect: perfective vs imperfective.

provjeriti – perfective: to check (once, as a complete action)
→ će provjeriti = will check (and finish checking).
provjeravati – imperfective: to be checking / to check repeatedly / to be in the process of checking
→ će provjeravati = will be checking (for some time, or repeatedly).

In this sentence, the idea is that the repairman will perform one complete check of the roof tomorrow. That’s why provjeriti (perfective) is the natural choice.

You would use provjeravati if, for example, he’ll be checking it over a longer period or doing it repeatedly:

Isti majstor će sutra cijeli dan provjeravati krov.
The same repairman will be checking the roof all day tomorrow.

What case is krov, and why doesn’t its form change?

krov here is in the accusative singular, because it’s the direct object of the verb provjeriti (check what? → the roof).

For many masculine inanimate nouns in Croatian, the nominative singular and accusative singular look exactly the same:

Nominative: krov je star – the roof is old
Accusative: provjeriti krov – to check the roof

So there is a case change, but it’s not visible in the form of the noun.

Do we need the comma before jer, and can we use jer and zato što in the same way?

Comma before jer
Yes, in standard writing you normally put a comma before jer when it introduces a clause of reason:

Isti majstor će sutra provjeriti krov, jer mislimo da malo prokišnjava.

jer vs. zato što

Both often translate as because, but there are nuances:

jer – simple, neutral because, very common in speech and writing.
Razgovaramo tiho, jer beba spava. – We’re talking quietly because the baby is sleeping.
zato što – a bit more emphatic: for the reason that / because of the fact that.
Often appears with zato earlier in the sentence:
Ne izlazimo na krov zato što mislimo da prokišnjava. – We’re not going onto the roof because we think it’s leaking.

In most everyday situations, you can swap jer and zato što without changing the meaning much:

… jer mislimo da malo prokišnjava.
… zato što mislimo da malo prokišnjava.

Why do we need da in mislimo da malo prokišnjava? In English we can say We think it leaks a bit without that.

In Croatian, when a verb like misliti (to think) takes a full sentence as its object, you almost always need a conjunction, most commonly da.

The pattern is:

mislimo + da + [full clause]

So:

Mislimo da malo prokišnjava.
= We think (that) it leaks a bit.

Without da, this is ungrammatical:

✗ Mislimo malo prokišnjava. → wrong

You could replace da with kako in some styles/dialects:

Mislimo kako malo prokišnjava. but the safe, standard form is mislimo da…

What does malo modify here: the verb prokišnjava, or something else?

Here malo functions as an adverb that modifies the verb prokišnjava:

malo prokišnjava ≈ leaks a little / leaks slightly

So it expresses the degree of leaking – it’s not a little roof, it’s leaking a little. You could paraphrase it as:

pomalo prokišnjava – it leaks a bit / somewhat
samo malo prokišnjava – it only leaks a little

What is the difference between prokišnjava and verbs like prokišnjavati or procuriti?

prokišnjava is the 3rd person singular present of the verb prokišnjavati (imperfective aspect).

prokišnjavati (impf.) – to be leaking, to leak (in general / repeatedly / as a state)
→ krov prokišnjava = the roof leaks / is leaking (as a continuing problem)

A corresponding perfective verb describing the event of starting / leaking once is often:

procuriti or prokišnjeti (depending on context and region)

Examples:

Voda je procurila kroz krov. – Water leaked through the roof (once / at some moment).
Krov već dugo prokišnjava. – The roof has been leaking for a long time. (ongoing state)

In your sentence:

… mislimo da malo prokišnjava.
describes an ongoing property of the roof: it (tends to) leak a little, not just once.

Why is mislimo in the present tense if the check will happen tomorrow?

Because it describes what we think right now, not what we will think tomorrow.

mislimo = we think (now, at the time of speaking)
će provjeriti = will check (tomorrow)

So the time frames are:

present: we currently think it leaks a bit → mislimo
future: the same repairman will check it tomorrow → će provjeriti

Saying mislit ćemo da malo prokišnjava would mean we will think (in the future) that it leaks a bit, which is not what is meant here.

Could we say Isti će majstor sutra provjeriti krov? Where exactly can će go?

Yes, Isti će majstor sutra provjeriti krov is perfectly correct.

In Croatian, the auxiliary će (future tense) is a clitic and tends to appear in second position. It can go:

after the whole first phrase:
Isti majstor će sutra provjeriti krov.
or after the first word of that phrase:
Isti će majstor sutra provjeriti krov.

Both are common and natural. Splitting isti and majstor is fine: they still clearly belong together as isti majstor in meaning.

If the repair person were female, how would the sentence change?

You would change the noun and adjective to the feminine form:

Ista majstorica će sutra provjeriti krov, jer mislimo da malo prokišnjava.

Changes:

isti → ista (feminine form of the same)
majstor → majstorica (female repair person)

Everything else in the sentence stays the same.

Could we omit malo and just say … jer mislimo da prokišnjava? Would that change the meaning?

Yes, you can omit malo:

… jer mislimo da prokišnjava.

This changes the meaning slightly:

mislimo da malo prokišnjava – we think it leaks a little / slightly
→ suggests a small problem.
mislimo da prokišnjava – we think it leaks
→ states the problem more generally or possibly more seriously, without softening it with a little.