Kad mi ga pošalješ, odmah ću ga pročitati i reći ću ti što mislim.

Yes, here kad means when or once.

• Kad is the shorter, very common form of kada.
• In this sentence, it introduces a time clause: when you send it to me.
• Ako means if, so it would sound more conditional or uncertain.

So:

• Kad mi ga pošalješ... = When/Once you send it to me...
• Ako mi ga pošalješ... = If you send it to me...

The original sentence suggests the speaker expects this to happen, so kad is the natural choice.

This is very common in Croatian.

After kad and similar time words, Croatian often uses the present tense of a perfective verb to talk about a future completed action.

So:

• pošalješ is formally present tense
• but in this structure it means when you send / once you have sent

Croatian does this where English often uses a present form too:

• When you send it, I’ll read it.

What matters is that pošalješ comes from a perfective verb, so it points to a single completed action.

Croatian usually leaves subject pronouns out when the verb ending already makes the subject clear.

Here the verb forms tell you who is doing the action:

• pošalješ = you send
• ću ... pročitati = I will read
• mislim = I think

So Croatian does not need to say ti or ja unless there is emphasis or contrast.

For example:

• Kad mi ga pošalješ... = When you send it to me...
• Kad mi ga ti pošalješ... would add emphasis, like when you send it to me and not someone else.

These are short unstressed pronouns, often called clitics.

• mi = to me
• ga = it or him, depending on context
• ti = to you

So:

• Kad mi ga pošalješ = When you send it to me
• reći ću ti = I’ll tell you

In this sentence, ga is most naturally understood as it, referring back to some previously mentioned thing.

Because Croatian short pronouns follow a fairly strict clitic order.

When both appear together, the usual order is:

• dative first: mi, ti, mu, etc.
• then accusative: ga, je, ih, etc.

So:

• mi ga = correct
• ga mi = not the normal order here

This is part of a broader Croatian rule that short clitics tend to cluster together in a fixed sequence.

Because it belongs to two different verbs:

• pošalješ ga = send it
• pročitati ga = read it

Croatian normally repeats the object pronoun when a new verb also needs that object.

So the sentence says, in effect:

• When you send it to me, I’ll read it right away...

English can sometimes feel lighter here, but Croatian very naturally repeats the pronoun where needed.

This is a question of aspect, which is very important in Croatian.

Here the sentence uses perfective verbs because it is talking about completed actions:

• pošalješ from poslati = send, as a completed act
• pročitati = read through, finish reading

That fits the meaning:

1. you send it
2. I read it completely
3. I tell you what I think

If you used the imperfective forms:

• šalješ
• čitati

the focus would shift toward an ongoing, repeated, or non-completed action, which is not what this sentence is aiming for.

Because ću is a clitic, and clitics usually go in the second position of their clause.

In:

• odmah ću ga pročitati

the first element is odmah, so ću comes right after it.

In:

• i reći ću ti

after i, the first real stressed word is reći, so ću follows that.

So the position of ću is not random. It depends on Croatian clitic placement rules.

A useful way to think about it:

• ću does not simply sit next to the English-style verb position
• it goes where Croatian clitics normally go

Yes, it can often be left out.

You could also say:

• Kad mi ga pošalješ, odmah ću ga pročitati i reći ti što mislim.

That is perfectly natural.

Repeating ću in:

• ...i reći ću ti...

makes the second future verb feel a bit more separate or balanced. It can sound slightly more deliberate or emphatic, but both versions are normal.

Because što mislim means what I think.

Here što introduces the content of what will be said:

• reći ću ti što mislim = I’ll tell you what I think

If you used da, the structure would be different and would not mean the same thing here.

So:

• što = what
• da = often that / to, depending on structure

Also, Croatian often leaves out the subject pronoun here, so što mislim is more natural than što ja mislim, unless you want special emphasis on I.

Yes.

Kad mi ga pošalješ is a subordinate clause, and when it comes before the main clause, Croatian normally separates it with a comma:

• Kad mi ga pošalješ, odmah ću ga pročitati...

That comma is standard and expected in normal writing.