Learn Hardcore Croatian: Moram poslati ručnik natrag, jer je mrlja ostala. - I have to send the towel back, because the stain remained.

Moram means I must / I have to. The dictionary form is morati (to have to / must).
It’s conjugated for 1st person singular: (ja) moram. Croatian often drops the subject ja because the verb ending already shows the person.

Why do we say poslati and not some other form like šaljem?

After morati, Croatian typically uses the infinitive: moram + infinitive → moram poslati (I have to send).
You can also use da + present in some contexts/regions (moram da pošaljem), but in standard Croatian the infinitive (moram poslati) is the most common/neutral choice.

What case is ručnik in, and why?

Ručnik is in the accusative singular because it’s the direct object of poslati (to send).
For masculine inanimate nouns like ručnik, the accusative singular is the same as the nominative singular, so it looks unchanged: ručnik.

Is natrag the same as nazad? Which one should I use?

In this sentence, natrag = back (as in “send it back”). Nazad is also commonly used with a very similar meaning.
In standard Croatian, natrag is often presented as the more “standard/neutral” option, but in everyday speech you’ll hear both:

Moram poslati ručnik natrag.
Moram poslati ručnik nazad.

Why is there a comma before jer?

Because jer (because) introduces a dependent clause, and Croatian standard punctuation uses a comma before jer:
..., jer je mrlja ostala.
So the comma is expected in formal and standard writing.

What does jer require grammatically—does it affect word order?

Jer introduces a subordinate clause but Croatian word order remains fairly flexible. The clause here is in a normal neutral order:
jer je mrlja ostala → because the stain remained.
You could also rearrange for emphasis (less neutral): jer je ostala mrlja, but the given version is perfectly natural.

Why is it je and not jest?

Je is the clitic (short) present form of biti (to be) used very often in sentences: je, sam, si, smo, ste, su.
Jest exists, but it’s typically used for emphasis/contrast (roughly “it is”):

Neutral: jer je mrlja ostala
Emphatic: jer jest mrlja ostala (rare/marked)

What case is mrlja, and why?

Mrlja is nominative singular because it’s the subject of the verb ostala (remained).
The structure is: mrlja (subject) + je (auxiliary) + ostala (past participle).

Why is it ostala (feminine) and not ostao/ostalo?

Because the subject mrlja is feminine singular, and past participles agree in gender and number:

masculine sg: ostao
feminine sg: ostala
neuter sg: ostalo
So mrlja je ostala is required.

Is je ostala a “past tense”? How is that formed?

Yes. This is the Croatian perfect tense (often used for past events). It’s formed with:
present of biti (auxiliary) + past participle
Here: je + ostala → has remained / remained.
Croatian uses the perfect very widely where English might choose either “remained” or “has remained.”

Can I omit je and just say jer mrlja ostala?

In standard Croatian, you normally keep the auxiliary: jer je mrlja ostala.
Dropping je can happen in some styles (especially informal, poetic, or certain dialect/register contexts), but it’s not the safe default for learners.

Does ostati mean “stay” or “remain,” and how is it different from ostavljati/ostaviti?

Ostati = to remain / stay (end up still there), focusing on the state after something happens: the stain is still there.
Ostaviti / ostavljati = to leave (something behind) / to let (something remain), focusing on an agent causing it: “Someone left a stain.”
So for “the stain didn’t come out; it’s still there,” ostati is the right idea.

If I want to say “I have to send the towel back because the stain is still there,” is there a present-tense option?

Yes. You could say: jer je mrlja još tu (because the stain is still here/there) or jer je mrlja još uvijek tu (still).
But the given sentence jer je mrlja ostala emphasizes the result after washing/cleaning: the stain “remained.”

Where does natrag go in the sentence—could it be elsewhere?

Yes, adverbs are flexible. These are all possible, with slightly different emphasis:

Neutral: Moram poslati ručnik natrag.
Emphasizing “back”: Moram ručnik poslati natrag.
More marked: Natrag moram poslati ručnik.
The original is very natural.

Is ručnik always masculine? What’s its plural?

Yes, ručnik is masculine.
Plural: ručnici (nominative plural).
Some useful forms:

Genitive sg: ručnika
Accusative pl (inanimate): ručnike (often same as nominative/genitive patterns depending on declension, but here it’s ručnike)

How would I say “I need to send the towel back…” instead of “I must…”?

A common alternative is Moram is already “have to/need to” in many contexts, but if you want a softer “need,” you can use:

Trebam poslati ručnik natrag... = I need to send the towel back...
Moram sounds more like obligation/necessity; trebam can feel more like practical need.

Is this sentence more like “I must” or “I have to” in tone?

Moram often corresponds to English I have to in everyday situations (practical necessity), not necessarily dramatic “must.” Context decides: returning a product because of a stain is a typical moram situation.