Η αδερφή μου προτιμά βερίκοκο και δαμάσκηνο, αλλά εγώ θέλω λίγο ανανά μετά το φαγητό.

Because εγώ adds contrast and emphasis.

Greek verb endings usually already show the subject, so θέλω by itself already means I want. But in this sentence, the speaker is contrasting their own preference with their sister’s:

• Η αδερφή μου προτιμά... αλλά εγώ θέλω...
• My sister prefers... but I want...

So εγώ is there for but I.

In Greek, unstressed possessive words like μου (my) usually come after the noun:

• η αδερφή μου = my sister
• το βιβλίο μου = my book

This is the normal pattern in everyday Greek.

Yes. Both mean sister.

• αδελφή is the more traditional/spelling-based form
• αδερφή is very common in everyday modern spoken Greek

A learner will see both. In casual modern usage, αδερφή is extremely common.

Because Greek often uses the singular when talking about a food item in a general sense, especially with verbs like prefer, eat, want, and so on.

So:

• προτιμά βερίκοκο και δαμάσκηνο

means she prefers apricot and plum as foods/flavors/types of fruit, not necessarily one single apricot and one single plum.

English often uses plural in similar contexts, but Greek does not always match that pattern.

Because Greek can omit the article when talking about foods in a general or non-specific way.

Here the fruits are not specific individual items like the apricot on the table. They are just being mentioned as kinds of food:

• προτιμά βερίκοκο και δαμάσκηνο
• θέλω λίγο ανανά

This sounds natural in Greek. If you add an article, the meaning can become more specific or slightly different in tone, depending on context.

They are direct objects, so they are in the accusative case.

• προτιμά βερίκοκο και δαμάσκηνο
• θέλω λίγο ανανά

A useful detail:

• βερίκοκο and δαμάσκηνο are neuter nouns, and for many neuter nouns the nominative and accusative look the same.
• ανανάς is a masculine noun. Its dictionary form is ο ανανάς, but the accusative singular is ανανά.

So:

• nominative: ο ανανάς
• accusative: τον ανανά / ανανά

Because λίγο here means a little, some, or a small amount of.

So:

• θέλω λίγο ανανά = I want a little pineapple / some pineapple

This is different from:

• θέλω έναν ανανά = I want one pineapple

In other words:

• λίγο ανανά = some pineapple as food
• έναν ανανά = one whole pineapple

Because the basic form of the noun is ο ανανάς, but here it is in the accusative singular, where the ending changes:

• nominative: ο ανανάς
• accusative: τον ανανά

When the article is omitted, you still keep the accusative form:

• θέλω λίγο ανανά

So the missing -ς is not random; it shows the noun is not in the dictionary form.

Because μετά can be used directly before a noun phrase in Modern Greek:

• μετά το φαγητό = after the meal / after food

This is very common and natural.

You may also hear μετά από..., especially in other contexts, and it is also correct. But in a phrase like this, μετά το φαγητό is perfectly normal and very common.

Literally, το φαγητό means the food or the meal. In this sentence, μετά το φαγητό most naturally means:

• after the meal
• after eating

So it is a very natural everyday expression.

They are both in the present tense.

• προτιμά = he/she prefers
• θέλω = I want

In this sentence:

• Η αδερφή μου προτιμά... = My sister prefers...
• εγώ θέλω... = I want...

These are simple present forms describing current preference or a general preference.

Yes. Greek word order is more flexible than English word order, because the endings and articles help show grammatical function.

For example, this sentence could be rearranged for emphasis:

• Μετά το φαγητό, εγώ θέλω λίγο ανανά.
• Η αδερφή μου βερίκοκο και δαμάσκηνο προτιμά.
  This is less neutral, but still possible in the right context.

The original version is a natural, neutral way to say it:

• Η αδερφή μου προτιμά βερίκοκο και δαμάσκηνο, αλλά εγώ θέλω λίγο ανανά μετά το φαγητό.