Cartonul este în cutie.

Because Romanian usually puts the definite article at the end of the noun, not before it.

• carton = a carton / cardboard
• cartonul = the carton / the cardboard

The ending -ul is a common definite article ending for masculine/neuter singular nouns.

A useful thing to know: carton is a neuter noun, and neuter nouns in Romanian behave like masculine in the singular and feminine in the plural. That is why you get cartonul in the singular.

This is a very common thing that confuses English speakers.

After prepositions like în (in), Romanian often uses a bare noun for locations, especially in ordinary everyday contexts:

• în cutie
• pe masă
• în pat
• la școală

So în cutie can naturally mean something like in the box or simply in a box, depending on context.

If you want to be more explicit:

• într-o cutie = in a box
• în cutia ... = in the box ...
• în cutia mare = in the big box
• în cutia de pe masă = in the box on the table

So the version in your sentence is normal Romanian.

Este means is. It is the 3rd person singular form of a fi (to be).

Yes, in everyday Romanian, you can very often say:

• Cartonul este în cutie.
• Cartonul e în cutie.

Both are correct.

A rough difference:

• este = a bit fuller, sometimes slightly more formal or careful
• e = very common in normal speech

Unlike in some other languages, you normally do not omit this verb in a regular sentence. So Cartonul în cutie would sound incomplete in standard Romanian.

This is the most neutral, straightforward word order:

subject + verb + place expression

• Cartonul = the subject
• este = the verb
• în cutie = where it is

Romanian word order is fairly flexible, though. You could also hear:

• În cutie este cartonul.

That version puts more focus on where it is, or creates a more marked style. But Cartonul este în cutie is the most neutral way to say it.

În is pronounced with a vowel that does not really exist in English.

The sound written î (or â inside words) is a close central vowel. A simple learner-friendly tip is:

• keep your tongue high, as if you were going to say ee
• but do not spread your lips much
• keep the sound more central and muted

So în is roughly like ihn, but that is only an approximation.

Also, notice that în means in / inside.

A useful approximation is:

• cutie → koo-TEE-eh
• este → often roughly YES-te

A few details:

• cutie has three syllables: cu-ti-e
• the final -ie is not like English -ee; it is pronounced separately
• este may sound like it begins with a y sound in normal speech

So the whole sentence can sound roughly like:

kar-TO-nul YES-te oon koo-TEE-eh

That is only an approximation, but it is a helpful starting point.

Yes, it matters, because gender affects articles and other forms.

• carton is neuter
• cutie is feminine

That is why their forms look different:

• un carton / cartonul
• o cutie / cutia

For neuter nouns like carton:

• singular behaves like masculine
• plural behaves like feminine

So:

• un carton
• două cartoane

This is one of the characteristic features of Romanian grammar.

Because în is used when something is inside something.

• în cutie = in the box / inside the box

Compare:

• pe cutie = on the box
• la cutie = usually not the natural choice for this meaning

So if the object is contained by the box, în is the correct preposition.

Not always. Romanian carton often refers to cardboard or stiff card/pasteboard, and in some contexts it can also refer to a carton/box-like item.

So you should not assume it always matches English carton perfectly. The exact meaning depends on context.

In this sentence, you should follow the meaning that was given to you, but it is good to know that carton and English carton are not always perfect one-to-one equivalents.