Ona kaže da ima dobar razlog, ali meni još ne kaže koji.

Yes. Serbian often drops subject pronouns because the verb form already shows the person and number.

• Kaže already means she/he says
• So Kaže da ima dobar razlog... is possible if the subject is clear from context

Using ona adds:

• clarity, if several people are being discussed
• emphasis or contrast, as in She says...

So ona is not grammatically necessary, but it is perfectly natural.

Here da introduces a subordinate clause, like English that:

• kaže da ima... = she says that she has...

In English, that is often omitted:

• She says she has...

In Serbian, da is usually kept in this kind of sentence after verbs like:

• kaže = says
• misli = thinks
• zna = knows
• veruje = believes

So da is a very common connector in Serbian.

Yes, depending on the structure.

In this sentence:

• Ona kaže da ima dobar razlog = She says that she has a good reason
• meni još ne kaže koji = she still doesn't tell me which one

So the same Serbian verb kazati / kaže can correspond to English say or tell, depending on whether the sentence focuses on:

• the content of what is said
• the person being told something

English separates these more strictly than Serbian does.

It is in the accusative.

The verb imati takes a direct object, so razlog is in the accusative case. But for a masculine inanimate singular noun like razlog, the accusative looks exactly the same as the nominative.

So:

• nominative: dobar razlog
• accusative: dobar razlog

They are identical in form here.

This is normal for masculine inanimate nouns in Serbian.

Both are forms of I / me in the dative, but they are used differently.

• mi = unstressed clitic form
• meni = stressed full form

In this sentence, meni sounds more emphatic or contrastive:

• ali meni još ne kaže koji = but she still doesn't tell me which one

It can suggest something like:

• to me, at least, she still doesn't say which one
• but she doesn't tell me

A more neutral version would be:

• ali mi još ne kaže koji

So meni is chosen for emphasis.

Here još means still or yet, depending on how you translate it in English.

• još ne kaže = still doesn't say / hasn't said yet

In negative sentences, još often gives the idea that something has not happened up to now, but may happen later.

So:

• meni još ne kaže koji = she still doesn't tell me which one
• or she hasn't told me which one yet

Both capture the idea well.

Because the noun it refers to is understood and does not need to be repeated.

Here koji stands for:

• koji razlog = which reason

So:

• ne kaže koji literally means she doesn't say which
• more naturally in English: she doesn't say which one

Serbian often omits a noun when it is already obvious from the context.

Because it agrees with the omitted noun razlog.

• razlog is masculine singular
• so the matching form is koji

If the hidden noun were different, the form would change:

• koja for a feminine singular noun
• koje for a neuter singular noun
• koji / koje / koja etc. in plural, depending on gender and case

So koji is not random: it matches razlog.

Because koji already introduces an indirect question.

Compare:

• kaže da ima dobar razlog = a that-clause
• ne kaže koji = an indirect question meaning doesn't say which one

So the second part is not built with da. It is built with the question word koji.

That is why:

• da ima is correct
• koji is correct
• but da koji would not work here

Yes, Serbian word order is fairly flexible.

This order is natural because:

• ali introduces contrast: but
• meni is placed early for emphasis: to me
• još ne kaže keeps još close to the negated verb

You may also hear:

• ali mi još ne kaže koji

That version sounds a bit more neutral because mi is the unstressed clitic form.

So the chosen order helps highlight meni.

Because ali connects two clauses:

• Ona kaže da ima dobar razlog
• ali meni još ne kaže koji

In Serbian, it is standard to put a comma before ali when it joins two clauses, just like English often uses a comma before but in similar sentences.