Δεν μου αρέσει η κέτσαπ στο σουβλάκι, προτιμώ τη μουστάρδα.

In Greek, μου αρέσει literally means it is pleasing to me.

So the structure is different from English:

• μου = to me
• αρέσει = pleases / is pleasing

That means:

• Δεν μου αρέσει η κέτσαπ = Ketchup is not pleasing to me = I don’t like ketchup

This is one of the most important patterns in Greek. Compare:

• Μου αρέσει ο καφές. = I like coffee.
• Μου αρέσουν τα φρούτα. = I like fruit / fruits.

So Greek often expresses liking as something that pleases you, rather than something you actively do.

μου is the weak form of εγώ in the sense of to me / my, depending on context. Here it means to me.

In this sentence:

• Δεν μου αρέσει... = ...is not pleasing to me

So μου is an indirect object pronoun.

A few similar forms are:

• μου = to me
• σου = to you
• του / της = to him / her
• μας = to us
• σας = to you (plural/formal)
• τους = to them

Example:

• Σου αρέσει η μουστάρδα; = Do you like mustard?

The verb agrees with the thing that is pleasing, not with the person who likes it.

Here, the subject is:

• η κέτσαπ = the ketchup

That is singular, so Greek uses:

• αρέσει = singular

Compare:

• Μου αρέσει η κέτσαπ. = I like ketchup.
• Μου αρέσουν οι σάλτσες. = I like sauces.

So:

• singular thing liked -> αρέσει
• plural things liked -> αρέσουν

In Greek, nouns need grammatical gender, even borrowed words.

κέτσαπ is a loanword, and in standard Greek it is usually treated as feminine, so you get:

• η κέτσαπ = the ketchup

Similarly:

• η μουστάρδα = the mustard

This is not something you can always predict from English; it is something you learn with the noun. It is best to memorize nouns together with their article:

• η κέτσαπ
• η μουστάρδα
• το σουβλάκι

Because μουστάρδα is the direct object of προτιμώ (I prefer), so it goes into the accusative case.

Greek articles change with case:

• nominative feminine singular: η
• accusative feminine singular: τη(ν)

So:

• η μουστάρδα = mustard, as a subject
• τη μουστάρδα = mustard, as an object

In this sentence:

• προτιμώ τη μουστάρδα = I prefer mustard

That is why the article changes.

Both are related forms of the same article/pronoun.

• full form: την
• shortened common form: τη

Before many consonants, Greek often drops the final -ν in everyday usage and writing:

• τη μουστάρδα is completely normal
• την μουστάρδα is also possible, but less common here

The final -ν is more often kept before vowels and certain consonants, or in more careful speech/writing.

For a learner, the safest thing is:

• recognize both τη and την
• don’t be surprised when the -ν disappears

στο is a contraction of:

• σε + το = στο

So:

• σε = in / on / at / to
• το = the
• στο = in the / on the / at the

Here:

• στο σουβλάκι = in/on the souvlaki

With food, Greek often uses σε in places where English may prefer on.

So the phrase means something like:

• on the souvlaki
• in the souvlaki wrap/pita depending on what exactly is meant.

Greek often uses the simpler preposition σε for food combinations and ingredients.

So:

• η κέτσαπ στο σουβλάκι naturally means ketchup on/in the souvlaki

If you say:

• πάνω στο σουβλάκι

that is more literally on top of the souvlaki and can sound more physically specific.

In everyday Greek, στο σουβλάκι is very natural when talking about what you put in or on that food.

Greek often uses the definite article with nouns in general statements much more than English does.

So Greek commonly says:

• Μου αρέσει ο καφές.
• Δεν τρώω το κρέας.
• Προτιμώ τη μουστάρδα.

Even when English would say:

• I like coffee.
• I don’t eat meat.
• I prefer mustard.

So the article here does not necessarily mean a very specific ketchup or a particular mustard. It can simply be the normal Greek way of talking about that item in general.

προτιμώ means I prefer.

It is a regular first-person singular verb form:

• προτιμώ = I prefer
• προτιμάς = you prefer
• προτιμά = he/she/it prefers
• προτιμάμε / προτιμούμε = we prefer
• προτιμάτε = you prefer
• προτιμούν / προτιμάνε = they prefer

In the sentence:

• προτιμώ τη μουστάρδα = I prefer mustard

Unlike μου αρέσει, this works more like English:

• subject = implied I
• object = τη μουστάρδα

Because Greek usually does not need to state the subject pronoun when the verb ending already shows who the subject is.

Here:

• προτιμώ already means I prefer

So εγώ is unnecessary unless you want emphasis or contrast.

Compare:

• Προτιμώ τη μουστάρδα. = I prefer mustard.
• Εγώ προτιμώ τη μουστάρδα. = I prefer mustard. / As for me, I prefer mustard.

Greek is a pro-drop language, so omitted subject pronouns are very common.

Greek often links short, closely related clauses with a comma, especially in everyday style.

So:

• Δεν μου αρέσει η κέτσαπ στο σουβλάκι, προτιμώ τη μουστάρδα.

means:

• I don’t like ketchup on souvlaki; I prefer mustard.

You could also say:

• Δεν μου αρέσει η κέτσαπ στο σουβλάκι, αλλά προτιμώ τη μουστάρδα.

But that sounds a little heavier, and αλλά (but) is not necessary here because the contrast is already clear.

Usually κέτσαπ is treated as an indeclinable loanword, meaning the noun itself does not change form.

So you may see:

• η κέτσαπ (nominative)
• την/τη κέτσαπ (accusative)

The article changes, but κέτσαπ itself stays the same.

This is common with many borrowed words in Greek.

Yes, absolutely.

That would sound perfectly natural:

• Δεν μου αρέσει η κέτσαπ. Προτιμώ τη μουστάρδα.

Using one sentence with a comma just makes it flow as one connected thought. Two shorter sentences are equally correct and may even be easier for a learner to produce.

Yes, they matter because Greek articles and sometimes adjective forms depend on gender.

In this sentence:

• η κέτσαπ -> feminine
• η μουστάρδα -> feminine
• το σουβλάκι -> neuter

That is why you get:

• η κέτσαπ
• τη μουστάρδα
• το σουβλάκι -> στο σουβλάκι

Memorizing nouns with their article is one of the best habits in Greek, because it helps with both gender and case later.