Sestra mi ga je stavila u torbu da ga ne izgubim.

No. mi is the short dative form of ja, so it means to me or for me, not my.

In this sentence, it marks the person affected by the action:

• Sestra mi ga je stavila u torbu = My sister put it in the bag for me / in my bag

If you wanted to say my sister explicitly, you would use moja sestra.

Because the same object is involved in both parts of the sentence.

• ga after mi = the thing she put
• ga after da = the thing I might lose

So Croatian repeats the object pronoun in both clauses:

• she put it
• so that I don’t lose it

ga is a short unstressed object pronoun meaning him or it, depending on context.

Here it is in the accusative, because it is the direct object of both stavila and izgubim.

The longer, stressed form is njega.

Because mi, ga, and je are all clitics: short unstressed words that have special placement rules in Croatian.

They usually appear near the beginning of the clause, after the first stressed word. Their order is not translated word-for-word from English; it follows Croatian clitic ordering.

So Sestra mi ga je stavila... is the natural pattern here.

Because stavila agrees with the subject sestra, which is feminine singular.

In Croatian past forms, the participle agrees with the subject in gender and number:

• stavio = masculine singular
• stavila = feminine singular
• stavili / stavile = plural

So with sestra, stavila is the correct form.

It is the Croatian perfect tense, formed with:

• the auxiliary je
• the past participle stavila

So je stavila means put or has put, depending on context. In English, it is often translated simply as a past tense.

Because u can take different cases depending on meaning:

• u + accusative = movement into something
• u + locative = being in something

Here the idea is put into the bag, so Croatian uses the accusative:

• u torbu = into the bag

If it meant location, it would be:

• u torbi = in the bag

Because Croatian often uses da + present tense to express purpose:

• da ga ne izgubim = so that I don’t lose it

English might also use:

• so I won’t lose it
• so I wouldn’t lose it
• to avoid losing it

Croatian normally uses a full clause with da in this kind of sentence.

Because the subject of the second clause is different from the subject of the first clause.

• In the first clause, the subject is sestra: she put it
• In the second clause, the understood subject is I: so that I don’t lose it

So izgubim is first person singular: I lose.

Because Croatian often leaves subject pronouns out when the verb ending already shows who the subject is.

The ending -im already tells you it is I:

• izgubim = I lose

You could add ja for emphasis, but it is not necessary.

After da, Croatian normally uses a present-form verb, even when English might prefer would in translation.

So:

• da ga ne izgubim literally looks like so that I don’t lose it
• but in natural English, depending on context, it can also be so I wouldn’t lose it

This is a normal difference between the two languages.

Because staviti is perfective: it describes one completed action.

That fits this sentence well:

• she put it in the bag once, as a completed act

stavljati is imperfective and would suggest repeated, ongoing, or habitual action.

So:

• stavila = completed act of putting
• stavljala = was putting / used to put / kept putting

Because Croatian has no articles.

So nouns like sestra and torbu do not need separate words for the or a. Whether the meaning is definite or indefinite is understood from context.

That is why Croatian can simply say:

• Sestra ... u torbu without adding anything equivalent to the or a.