Još nisam našla ključ.

Here još means still or yet, depending on how you translate the whole sentence into natural English.

So:

• Još nisam našla ključ. = I still haven’t found the key.
• very naturally also: I haven’t found the key yet.

With a negative verb, još often gives that still not / not yet idea.

Because Serbian uses special negative forms of jesam (to be, used as an auxiliary in the past tense).

For I am not / I have not, the form is:

• nisam

Other forms are:

• nisi = you are not / you have not
• nije = he/she/it is not / has not
• nismo = we are not / have not
• niste = you plural/formal are not / have not
• nisu = they are not / have not

So nisam našla is the normal correct structure.

Yes, in Serbian this is a past tense form: the perfect.

It is built with:

• the auxiliary jesam in the present tense, here negative nisam
• plus the past participle, here našla

So literally, structurally, Serbian is using its normal past construction.

In English, though, the most natural translation is often the present perfect:

• I haven’t found the key yet / still haven’t found the key

That is because the action is relevant up to now, not just at some finished point in the past.

Because the past participle in Serbian agrees with the subject’s gender.

• našla = feminine singular
• našao = masculine singular

So this sentence tells you that the speaker is female.

Compare:

• Još nisam našla ključ. = said by a woman
• Još nisam našao ključ. = said by a man

The dictionary form is naći, meaning to find.

This verb is a bit irregular-looking, so the forms can seem surprising at first:

• infinitive: naći
• past participle masculine: našao
• past participle feminine: našla

So našla is simply the feminine past participle of naći.

Because Serbian often drops subject pronouns when they are not needed.

The form nisam already tells you the subject is I, and našla also adds that the speaker is feminine.

So Ja is not necessary.

You could add it for emphasis:

• Ja još nisam našla ključ.

That would sound more like I still haven’t found the key with extra emphasis on I.

Here ključ is the direct object, so it is in the accusative case.

However, ključ is a masculine inanimate noun, and for many masculine inanimate nouns:

• nominative singular = accusative singular

So:

• nominative: ključ
• accusative: ključ

That is why it looks unchanged.

A useful contrast:

• masculine inanimate often stays the same in the accusative
• masculine animate often changes

For example:

• Vidim ključ. = I see the key.
• Vidim čoveka. = I see the man.

This is a very common and very good question.

In affirmative sentences, the auxiliary sam is a clitic, and clitics usually go in the second position:

• Našla sam ključ. = I found the key.

But in negative sentences, the negative auxiliary form is:

• nisam

and it normally stands together as one unit before the participle:

• Nisam našla ključ.

So:

• affirmative: Našla sam ključ.
• negative: Nisam našla ključ.

That difference is normal Serbian grammar.

Yes, Serbian word order is fairly flexible, although some versions are more neutral than others.

All of these are possible:

• Još nisam našla ključ.
• Nisam još našla ključ.

Both are natural. The difference is mainly one of emphasis and rhythm.

Very roughly:

• Još nisam našla ključ. puts još first and makes the still/not yet idea prominent.
• Nisam još našla ključ. is also extremely common and natural.

If you move ključ earlier, you usually create stronger emphasis or contrast:

• Ključ još nisam našla. = It’s the key that I still haven’t found.

It can correspond to both, depending on context and translation style.

• Još nisam našla ključ. can be translated as:
  • I still haven’t found the key.
  • I haven’t found the key yet.

English often chooses between still and yet based on style, but Serbian još comfortably covers this area.

Yes. Pronaći also means to find, and:

• Još nisam pronašla ključ.

is perfectly natural.

Very broadly:

• naći = find
• pronaći = find, often a bit more explicit or complete-sounding

In many everyday contexts, they are very close in meaning. A learner should definitely understand both.