Danas ne mogu da putujem, jer nemam kartu.

In Serbian, subject pronouns are often omitted because the verb ending already shows the person.

• mogu = I can
• putujem = I travel / am traveling
• nemam = I do not have

So ja is not necessary. You would add ja only for emphasis or contrast, for example:

• Ja danas ne mogu da putujem, a on može.
  = I can’t travel today, but he can.

The sentence without ja sounds natural and normal.

Ne mogu is the negative form of mogu, which comes from the verb moći = to be able, can.

• mogu = I can
• ne mogu = I cannot / I can’t

It is:

• present tense
• 1st person singular
• used as a modal verb, like English can

So ne mogu da putujem literally means I cannot travel.

After modal verbs such as moći, Serbian very often uses:

• da + present tense

So:

• mogu da putujem = I can travel

This is one of the most common patterns in modern Serbian.

You may also encounter the infinitive:

• mogu putovati

That is also understandable and possible, but mogu da putujem is the more typical everyday Serbian wording.

Putujem is the 1st person singular present tense of the verb putovati = to travel.

So:

• putujem = I travel / I am traveling

In this sentence, it appears after da, but it is still a present-tense form:

• da putujem

This is normal in Serbian after modal verbs.

Also, putovati is an imperfective verb, which fits the general idea of traveling as an activity, rather than a single completed act.

This is a very common learner question.

Usually, ne is written separately from the verb:

• ne mogu
• ne znam
• ne radim

But some negative forms are written together, and nemam is one of them:

• imam = I have
• nemam = I do not have

So you simply have to learn that:

• ne mogu = two words
• nemam = one word

This is standard spelling.

Because kartu is the accusative singular form, and it is the direct object of nemam.

The noun is:

• karta = nominative singular

But after imati / nemati when it means to have / not have something, you use the direct object, so:

• nemam kartu = I don’t have a ticket

For many feminine nouns ending in -a, the accusative singular changes to -u:

• karta → kartu
• knjiga → knjigu
• torba → torbu

Serbian does not have articles like English a/an and the.

So kartu can mean:

• a ticket
• the ticket

The exact meaning depends on context.

In this sentence, English usually translates it as a ticket, but Serbian itself does not mark that difference with an article.

Yes, you could also use zato što.

• jer = because
• zato što = because

In this sentence:

• Danas ne mogu da putujem, jer nemam kartu.
• Danas ne mogu da putujem zato što nemam kartu.

Both are correct.

Jer is short, common, and natural.
Zato što is also very common and may sound a little fuller or more explicit, but the basic meaning here is the same.

Because jer introduces a dependent clause explaining the reason.

So the sentence has two parts:

• Danas ne mogu da putujem
• jer nemam kartu

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

So the comma here is correct and expected.

Yes, you can move it.

Danas is an adverb of time, and Serbian word order is fairly flexible. Putting Danas first gives the time frame immediately:

• Danas ne mogu da putujem, jer nemam kartu.

You could also say:

• Ne mogu danas da putujem, jer nemam kartu.

Both are natural. The difference is mostly about emphasis and flow.

• Danas... puts more focus on today
• Ne mogu danas... puts the negated ability first: I can’t today...

Here it means to travel in the sense of being able to make a trip today.

Because it follows ne mogu da..., the whole phrase means:

• ne mogu da putujem = I can’t travel

So even though putujem is formally a present-tense form, in this structure it works much like an English infinitive after can.

It does not necessarily mean that you are in the middle of traveling right now. It simply expresses the activity that you are unable to do today.

Yes. Putujem comes from putovati, which is imperfective and means to travel in a general sense.

If you wanted to emphasize a single completed departure, Serbian might use a perfective verb such as:

• otputovati
• da otputujem

For example:

• Danas ne mogu da otputujem, jer nemam kartu.

That sounds more like:

• I can’t leave / set off today, because I don’t have a ticket.

So:

• da putujem = more general to travel
• da otputujem = more specifically to depart / to go on the trip

The original sentence is perfectly natural if the idea is simply I can’t travel today.