Learn Hardcore Croatian: Subota je dan kad ne moram ulaziti u tramvaj koji je pun ljudi. - Saturday is the day when I don’t have to get into the tram that is full of people.

morati ≈ must / have to
- ne moram ≈ I don’t have to / I am not obliged to
trebati ≈ need
- ne trebam literally ≈ I don’t need (to), but in Croatian it often sounds like you lack a need, not that you’re free of obligation.

In this kind of sentence about obligation, natural Croatian is:

Subota je dan kad ne moram ulaziti u tramvaj…
- Saturday is a day when I don’t have to get on the tram…

If you say ne trebam ulaziti u tramvaj, it is understandable, but it can sound a bit off or non‑native in many contexts. Ne moram is the default choice for “I don’t have to” in the sense of “I’m not required to”.

Why is there no ja (I) in kad ne moram ulaziti? How do I know who the subject is?

Croatian does not need subject pronouns as often as English, because the verb ending shows the person:

(ja) ne moram = I don’t have to
(ti) ne moraš = you don’t have to
(on/ona/ono) ne mora = he/she/it doesn’t have to

In kad ne moram ulaziti, the ending -am tells you it’s 1st person singular (I). So adding ja is optional:

kad ne moram ulaziti… (normal, natural)
kad ja ne moram ulaziti… (also correct, but emphasizes I specifically)

Why is ulaziti used here instead of ući? What’s the difference?

Croatian has aspect: imperfective vs perfective verbs.

ulaziti – imperfective (focus on process, repeated or habitual action)
ući – perfective (focus on a single, completed action)

In the sentence:

Subota je dan kad ne moram ulaziti u tramvaj…

we’re talking about a regular, repeated situation (every Saturday, in general life). For habits and general statements, Croatian uses the imperfective:

ne moram ulaziti = I don’t have to (ever / generally) board/get on trams (on that day)

If you used ući here (ne moram ući u tramvaj), it would sound more like “I don’t have to enter the tram (this one time)”, which doesn’t fit the general, habitual meaning of the sentence.

Why is it u tramvaj and not u tramvaju?

The preposition u can be followed by:

accusative: movement into something (direction)
locative: being in something (location)

In our sentence we have movement:

ulaziti u tramvaj – to enter/get into the tram
(u
- accusative: tramvaj → u tramvaj)

For being inside, you would use the locative:

biti u tramvaju – to be in the tram
(u
- locative: tramvaj → u tramvaju)

So ulaziti u tramvaj is correct because you’re moving into the tram.

How does the pronoun koji work in tramvaj koji je pun ljudi? Why koji and not koja or koje?

Koji / koja / koje is a relative pronoun meaning which / that / who, and it must agree with the noun it refers to in:

gender
number
case

Tramvaj is:

masculine
singular
nominative (it’s the subject of the relative clause)

So we use:

tramvaj koji je pun ljudi
- koji = masculine singular nominative

If the noun were different, the form would change:

autobus (m.sg.): autobus koji je pun ljudi
škola (f.sg.): škola koja je puna ljudi
mjesto (n.sg.): mjesto koje je puno ljudi

In our sentence, koji clearly refers back to tramvaj.

Why is it pun ljudi and not something like pun s ljudima or pun ljudi ljudi?

The adjective pun (full) takes the genitive case, not a preposition:

pun
- genitive = full of …

So:

pun ljudi = full of people
(ljudi is genitive plural)

You do not say:

✗ pun s ljudima
✗ pun od ljudi (very unusual; you might hear it colloquially but pun ljudi is standard)

Also, you don’t repeat ljudi:

✗ pun ljudi ljudi – incorrect

So the correct structure is simply:

tramvaj koji je pun ljudi – a tram that is full of people

Could I change the word order, for example to Dan je subota kad… or Subota je kad ne moram ulaziti u tramvaj…?

Word order in Croatian is freer than in English, but not everything sounds natural.

The original:

Subota je dan kad ne moram ulaziti u tramvaj koji je pun ljudi.

This sounds neutral and natural: Subota is being identified as a day of type X.

Some alternatives:

Dan je subota kad ne moram ulaziti u tramvaj…
– Grammatically possible, but sounds strange; it suggests “The day is Saturday when…” which doesn’t match the intended meaning.
Subota je kad ne moram ulaziti u tramvaj koji je pun ljudi.
– Literally: “Saturday is when I don’t have to get on a tram that is full of people.”
– This can be used, but it feels more like a definition of “Saturday” and is a bit more colloquial and emphatic.

The original Subota je dan kad… is the most natural neutral formulation here.

Could I say ne moram se voziti tramvajem instead of ne moram ulaziti u tramvaj? Is there a difference?

Yes, you can, and it slightly changes the nuance:

ne moram ulaziti u tramvaj
– focuses on the act of getting on / boarding the tram
ne moram se voziti tramvajem
– focuses on riding / travelling by tram

In real usage, if you just mean “I don’t have to use the tram”, ne moram se voziti tramvajem is very natural and probably more common. Your sentence with that change:

Subota je dan kad se ne moram voziti tramvajem koji je pun ljudi.

Both are correct; the original just chooses to highlight the boarding action.

Why is the present tense used (ne moram ulaziti)? Does it mean this happens regularly?

Yes. In Croatian, the present tense is used for:

actions happening now
habitual or general truths

Here it clearly describes a habitual pattern:

On Saturdays, in general, I do not have to get on a crowded tram.

So the present tense ne moram ulaziti expresses a repeated, general situation, similar to English “Saturday is the day when I don’t have to…” rather than something happening just this one specific Saturday.