Στην πιατέλα έχει τυρί, ψωμί και ντομάτα.

Στην is a contraction of σε + την.

• σε = in / on / at
• την = the (feminine accusative singular)

So:

• στην πιατέλα = on the platter

Greek very often combines σε with the definite article:

• σε + το → στο
• σε + τη(ν) → στη(ν)
• σε + τον → στον

In this sentence, πιατέλα is feminine, so you get στην πιατέλα.

Because after σε, Greek normally uses the accusative.

The dictionary form is η πιατέλα (the platter), and in the accusative singular it becomes:

• την πιατέλα

With σε, that becomes:

• σε την πιατέλα
• contracted: στην πιατέλα

So even though English says on the platter, Greek uses σε + accusative.

Yes, έχει literally means has, but in everyday Greek it is also very commonly used to mean there is / there are, especially in informal spoken language.

So:

• Στην πιατέλα έχει τυρί...
  = There is cheese on the platter...

This is a very natural colloquial Greek structure.

A more formal or more explicitly existential version would be:

• Στην πιατέλα υπάρχουν τυρί, ψωμί και ντομάτα.

But in normal speech, έχει is extremely common.

Because this έχει is being used in an impersonal existential way, similar to English there is / there are.

In this use, Greek often keeps έχει in the singular even when several things follow:

• Έχει πολύ κόσμο. = There are a lot of people.
• Στο τραπέζι έχει ποτήρια και πιάτα. = There are glasses and plates on the table.

So the verb is not really agreeing with τυρί, ψωμί και ντομάτα as a normal subject in the usual way. It is more like a fixed expression meaning there is/are.

Because Greek often leaves out the article when listing foods or mentioning things in an indefinite, non-specific way.

So:

• τυρί, ψωμί και ντομάτα
  means something like cheese, bread, and tomato

This sounds like a simple list of what is present.

If you added articles, the meaning would become more specific:

• το τυρί, το ψωμί και η ντομάτα
  = the cheese, the bread, and the tomato

That would suggest particular, identifiable items.

Not necessarily. In food descriptions, Greek often uses the singular to name an ingredient or food item in a general way.

So:

• τυρί, ψωμί και ντομάτα
  can mean cheese, bread, and tomato as food items

This is similar to English menu-style wording, where singular nouns can name ingredients:

• cheese and tomato
• bread and butter

If the speaker wanted to emphasize several tomatoes, they could say:

• ντομάτες = tomatoes

But the singular here is perfectly natural.

In this sentence it means on the platter.

Greek σε covers several English prepositions depending on context, including:

• in
• on
• at

So you should not translate σε too mechanically. With something like πιατέλα, English naturally says on the platter, while Greek still uses σε:

• στην πιατέλα = on the platter

Greek word order is more flexible than English word order. Putting Στην πιατέλα first highlights the location:

• Στην πιατέλα έχει τυρί, ψωμί και ντομάτα.

This feels like:

• On the platter, there is cheese, bread, and tomato.

You could also hear other word orders, such as:

• Έχει τυρί, ψωμί και ντομάτα στην πιατέλα.

But starting with the location is very natural when you want to say where something is.

Not usually in the same natural way.

• έχει here means there is / there are
• είναι means is / are

If you say:

• Στην πιατέλα είναι τυρί, ψωμί και ντομάτα

it sounds less natural in ordinary Greek for simple existence. Native speakers would normally prefer έχει or υπάρχουν here.

So for there is/are on the platter, έχει is the everyday choice.

Not exactly.

• πιάτο = plate
• πιατέλα = platter / serving plate / large dish

So πιατέλα usually suggests a larger serving dish used to hold or present food, not just an ordinary individual plate.

That is why on the platter is a good translation here.

It is natural, everyday Greek.

Especially natural features are:

• στην instead of σε την
• έχει used to mean there is / there are
• the simple food list without articles

A textbook might sometimes prefer a more formal version with υπάρχουν, but the sentence as given sounds like normal spoken Greek.