Sutra putujem avionom, pa večeras spremam kofer.

In Serbian, the present tense is often used for planned or certain future actions, especially when a time word already makes the future clear.

Here, sutra already tells you the traveling happens tomorrow, so putujem naturally means I’m traveling tomorrow / I travel tomorrow.
Likewise, večeras makes it clear that spremam happens tonight.

So this is very natural Serbian:

• Sutra putujem avionom.
• Večeras spremam kofer.

You could also say Sutra ću putovati avionom, but the present tense often sounds more direct and natural for arrangements or plans.

Serbian usually does not need a subject pronoun when the verb ending already shows who is doing the action.

• putujem = I travel / I am traveling
• spremam = I prepare / I am packing

Because the -m ending shows 1st person singular, ja is unnecessary.

You can add ja only for emphasis or contrast:

• Ja putujem sutra, ne on. = I’m traveling tomorrow, not him.

So in normal speech, leaving out ja is completely standard.

Avionom is in the instrumental case. After verbs of travel, Serbian often uses the instrumental to show the means of transportation.

So:

• putujem avionom = I’m traveling by plane
• idem autobusom = I’m going by bus
• putujemo vozom = we’re traveling by train

For many masculine and neuter nouns, the instrumental singular often ends in -om.

So avion becomes avionom because it means by plane, not just plane.

Grammatically, yes, the instrumental case often has the broad idea of with / by means of, but in this sentence the natural English translation is by plane.

So:

• putujem avionom = I travel by plane not usually I travel with a plane

This is just how Serbian expresses transportation in many cases.

Here pa connects the two parts of the sentence and means something like:

• so
• and so
• therefore
• sometimes just and then

In this sentence:

• Sutra putujem avionom, pa večeras spremam kofer.

the idea is: I’m traveling by plane tomorrow, so tonight I’m packing my suitcase.

So pa shows a natural consequence or sequence: tomorrow’s trip is the reason for tonight’s packing.

Because kofer is the direct object of spremam, and it is a masculine inanimate noun.

In Serbian, the accusative singular of masculine inanimate nouns is usually the same as the nominative.

So:

• nominative: kofer
• accusative: kofer

That is why you say:

• spremam kofer = I’m packing the suitcase

Compare that with a masculine animate noun:

• vidim čoveka = I see the man

There the accusative changes.

No. Both sutra and večeras work as adverbs of time, so they can stand on their own without a preposition.

• sutra = tomorrow
• večeras = tonight / this evening

So you simply say:

• Sutra putujem.
• Večeras spremam kofer.

This is similar to English, where you also normally say tomorrow and tonight without a preposition.

Yes, Serbian word order is more flexible than English word order. The sentence starts with sutra and večeras because those time words are being highlighted first.

Current sentence:

• Sutra putujem avionom, pa večeras spremam kofer.

This feels natural and clear: first tomorrow, then tonight, showing the relationship between the two actions.

You could also say:

• Večeras spremam kofer, jer sutra putujem avionom.
• Putujem sutra avionom, pa večeras spremam kofer.

These are all possible, but they may shift the emphasis slightly. Serbian often puts time expressions early in the sentence.

They are very close, and in many situations both can be translated as I’m packing my suitcase.

But there can be a slight nuance:

• spremam kofer can mean I’m getting the suitcase ready
• pakujem kofer focuses more directly on putting things into it

In everyday speech, both are very common and often interchangeable in travel contexts.

So this sentence sounds perfectly natural with spremam kofer, but you may also hear:

• Večeras pakujem kofer.

Because the sentence presents these actions as planned activities/processes, not as single completed results.

• putovati is imperfective: to travel
• spremati / spremati se / spremam here is also imperfective in meaning: to prepare / to be packing

Imperfective verbs are very common in the present tense, including when the present tense is used for the future.

If you used a more result-focused form, the meaning would shift slightly toward completion rather than the ongoing activity or plan.

So here the imperfective forms sound natural and neutral.