U kofer ne stavljam mnogo odeće, jer ostajem samo dva dana.

Because u can mean either in or into, and the case tells you which meaning is intended:

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

Here the verb stavljam means I put / I am putting, which involves movement, so u kofer is correct: into the suitcase.

Compare:

• Stavljam odeću u kofer. = I’m putting clothes into the suitcase.
• Odeća je u koferu. = The clothes are in the suitcase.

In Serbian, the negative particle ne is normally written separately from verbs:

• ne stavljam = I am not putting / I don’t put
• ne idem = I’m not going
• ne znam = I don’t know

There are a few common exceptions where the negative form is written together, such as:

• nemam
• nisam
• neću

But with stavljam, the standard spelling is definitely ne stavljam.

This is a question of aspect, which is very important in Serbian.

• stavljati / stavljam = imperfective
• staviti / stavim = perfective

In this sentence, the speaker is talking about the process or general action of packing, not a single completed act. That is why Serbian uses the imperfective stavljam.

So:

• ne stavljam mnogo odeće = I’m not putting much clothing in / I don’t put much clothing in

Using ne stavim here would sound unnatural for this meaning. Perfective present forms usually do not describe an action happening right now in the same way English does.

It can mean either, depending on context.

The Serbian present tense often covers both:

• English simple present
• English present continuous

So ne stavljam mnogo odeće could mean:

• I’m not putting many clothes in the suitcase
• I don’t put many clothes in the suitcase

In this sentence, because of jer ostajem samo dva dana, the most natural interpretation is the current situation: the speaker is packing now, or talking about this particular trip.

After quantity words like mnogo, malo, puno, the following noun usually goes in the genitive.

That is why:

• mnogo odeće = a lot of clothes
• not mnogo odeća

The base form is odeća, and the genitive singular is odeće.

This happens because odeća is treated as a mass/collective noun, not as a normal countable plural noun.

Grammatically, odeća is singular. But in meaning, it refers to clothing/clothes in general.

That is very different from English:

• English: clothes is plural
• Serbian: odeća is singular

So you say:

• Odeća je skupa. = Clothes are expensive.
• literally: The clothing is expensive.

This is why the form in the sentence is mnogo odeće, not something built like an English plural.

Yes. Both jer and zato što can mean because.

So these are both possible:

• U kofer ne stavljam mnogo odeće, jer ostajem samo dva dana.
• U kofer ne stavljam mnogo odeće, zato što ostajem samo dva dana.

In many situations, jer is shorter and very natural in everyday speech. Zato što can sound a bit more explicit or slightly heavier, but both are correct here.

There are two things happening here.

1. Why dva and not dve?
Because dan is a masculine noun, and with masculine nouns you use dva.

• dva dana = two days

Dve is used with feminine nouns:

• dve knjige = two books

2. Why dana and not dani?
After numbers like dva, tri, and četiri, Serbian nouns usually take a special form often called the paucal form. For many nouns, this looks like the genitive singular.

So:

• jedan dan
• dva dana
• tri dana
• četiri dana
• pet dana

With expressions of duration, this is completely normal:

• ostajem dva dana = I’m staying for two days

Because samo modifies the duration phrase dva dana.

• samo dva dana = only two days

This is the most natural placement in Serbian. It directly highlights the small amount of time.

You could move words around in Serbian, but this version is the most straightforward and natural here.

Yes, it is natural. Serbian word order is much more flexible than English word order.

Starting with U kofer puts focus on the destination, as if saying:

• As for the suitcase, I’m not putting many clothes in it...

A more neutral order would also be possible:

• Ne stavljam mnogo odeće u kofer, jer ostajem samo dva dana.

Both are correct. The original sentence simply gives a bit more emphasis to u kofer.

Because jer introduces a subordinate clause giving the reason.

So the sentence has two parts:

• U kofer ne stavljam mnogo odeće
• jer ostajem samo dva dana

In standard Serbian punctuation, a clause introduced by jer is normally separated by a comma.

So the comma here is expected and correct.