Ostavila sam ključ na stolu.

Because the speaker is female.

In Serbian past tense, the main verb agrees with the gender and number of the subject:

• ostavila sam = I left (said by a woman)
• ostavio sam = I left (said by a man)

So ostavila sam ključ na stolu is something a woman would say.

Sam is the 1st person singular form of the auxiliary verb biti (to be) used to form the past tense.

So the past tense here is built like this:

• ostavila = past participle
• sam = I am auxiliary, which helps make the past tense

Together:

• ostavila sam = I left

This is a very common Serbian past-tense pattern.

Because Serbian often drops the subject pronoun when it is already clear from the verb form.

Here, sam already tells you the subject is I, so ja is unnecessary.

• Ostavila sam ključ na stolu. = normal, natural
• Ja sam ostavila ključ na stolu. = also correct, but adds emphasis to I

So if you include ja, it usually sounds more emphatic, like I left the key on the table.

In Serbian, short unstressed words like sam are clitics, and they usually come in the second position of the sentence or clause.

That is why you often see:

• Ostavila sam ključ na stolu.

Here, ostavila counts as the first element, and sam comes right after it.

You can also change the word order for emphasis:

• Ključ sam ostavila na stolu.
• Na stolu sam ostavila ključ.

These are also correct, but the neutral version is Ostavila sam ključ na stolu.

Because ključ is the direct object, and for inanimate masculine nouns, the accusative singular is usually the same as the nominative singular.

So:

• nominative: ključ = key
• accusative: ključ = key as a direct object

That is why you say:

• Ostavila sam ključ = I left the key

If it were a masculine noun referring to a living being, the accusative would usually look different.

Because na stolu expresses location: the key is on the table.

After na, Serbian uses different cases depending on meaning:

• na + locative = on/at a location
• na + accusative = movement onto something

So:

• na stolu = on the table (location)
• na sto = onto the table (movement toward it)

In this sentence, the key ended up resting on the table, so na stolu is the natural choice.

Stolu is locative singular of sto (table).

The noun changes because it comes after the preposition na when location is meant.

So:

• sto = dictionary form
• na stolu = on the table

This is a very important pattern in Serbian: prepositions often require specific cases.

Because na means on in the sense of being on a surface, while u usually means in/inside.

So:

• na stolu = on the table
• u stolu would mean something like in the table, which usually does not fit the intended meaning

Since a key is resting on top of the table, na is the correct choice.

Ostavila comes from the verb ostaviti, which is a perfective verb.

That means it presents the action as completed:

• ostaviti = to leave, to put down and leave behind, as a completed act

In this sentence, the action is seen as finished: the speaker left the key on the table.

A related imperfective verb is ostavljati, which would focus more on process, repetition, or habit rather than a single completed act.

Yes. Serbian word order is fairly flexible, and changing it usually changes emphasis, not the basic meaning.

For example:

• Ostavila sam ključ na stolu. = neutral
• Ključ sam ostavila na stolu. = emphasizes the key
• Na stolu sam ostavila ključ. = emphasizes on the table

So the original sentence is the most neutral and natural basic version, but other orders are possible depending on what you want to stress.