Pre nego što pošalješ pismo, proveri da li ima grešaka.

Pre nego što means before when it introduces a whole clause, that is, a subject + verb idea:

• Pre nego što pošalješ pismo = Before you send the letter

By itself, pre is usually used before a noun or noun phrase:

• pre ručka = before lunch
• pre sastanka = before the meeting

So:

• pre pisma would mean before the letter, which is not the same idea
• pre nego što pošalješ pismo is needed because Serbian is saying before you send the letter

This is about aspect, which is very important in Serbian.

• pošalješ comes from the perfective verb poslati
• šalješ comes from the imperfective verb slati

Here, Serbian uses pošalješ because the action is seen as one completed act: sending the letter.

So the sentence means:

• first, check
• then, send the letter

The sending is viewed as a single completed event, so the perfective verb is natural.

If you used šalješ, it would sound more like an ongoing or repeated action, which does not fit this situation as well.

In Serbian, after conjunctions like pre nego što, kad, ako, and similar words, the present tense is often used to refer to a future action.

So:

• pre nego što pošalješ pismo literally looks like before you send the letter
• but it refers to something that will happen later

This is normal Serbian grammar. English often also does something similar:

• Before you send the letter, check...

English does not say before you will send the letter, and Serbian works similarly here.

Because pismo is the direct object of pošalješ, so it is in the accusative case.

The important detail is that pismo is a neuter singular noun, and for many neuter nouns:

• nominative singular = accusative singular

So:

• pismo = nominative singular
• pismo = accusative singular too

That is why the form does not change.

Proveri is the imperative, singular, from proveriti.

It means:

• check!

The sentence is giving an instruction to one person in an informal singular way, like English check.

Why this form?

• proveri = singular imperative
• proverite = plural or polite imperative
• proveravaj = imperfective imperative, more like keep checking / check regularly

Here the idea is a single complete check, so proveri fits very well.

Here da li means whether / if in an indirect yes-no question.

So:

• proveri da li ima grešaka = check whether there are mistakes

This is not the same if as in a condition like if it rains. In Serbian:

• da li introduces a yes-no content question
• ako usually introduces a condition

So:

• proveri da li ima grešaka = check if/whether there are mistakes
• not conditional if

A very common alternative is:

• proveri ima li grešaka

That means the same thing and is also very natural.

Because ima here does not simply mean has. It is being used in an existential way, like English there is / there are.

So:

• ima grešaka = there are mistakes or there are some mistakes
• nema grešaka = there are no mistakes

In this kind of expression, Serbian commonly uses ima in the same fixed way, even when the following noun idea is plural.

So the sentence is not really saying it has mistakes in the ordinary ownership sense. It is using a structure that means there exist mistakes.

Grešaka is the genitive plural of greška.

After existential ima / nema, Serbian very often uses the genitive, especially when talking about an indefinite amount or simply asking whether any such things exist.

So:

• ima grešaka = there are mistakes / any mistakes
• nema grešaka = there are no mistakes

This is a very common Serbian pattern.

Compare:

• Ima jednu grešku. = It has one mistake.
  Here you have a specific counted object, so the structure is different.

But in your sentence, the idea is general: Are there any mistakes? That is why grešaka is used.

Yes, in this sentence the comma is normal and expected.

The clause:

• Pre nego što pošalješ pismo

comes first, and then the main instruction follows:

• proveri da li ima grešaka

When a subordinate clause comes before the main clause, Serbian normally separates it with a comma.

So:

• Pre nego što pošalješ pismo, proveri da li ima grešaka.

is the standard punctuation.

Yes. Serbian word order is fairly flexible, although some versions sound more natural than others.

For example, you could also say:

• Proveri da li ima grešaka pre nego što pošalješ pismo.

This means the same thing.

The original version puts the before clause first, which highlights the timing right away:

• Before you send the letter, ...

That is very natural when giving instructions.

Yes. Both are natural.

• proveri da li ima grešaka
• proveri ima li grešaka

The second version is a bit more compact. Learners often first meet da li because it is very clear and easy to recognize, but verb + li is also common and important.

So these are both good Serbian:

• Proveri da li ima grešaka.
• Proveri ima li grešaka.

The meaning is the same: Check whether there are mistakes.