Za večeru želim bananu i čaj, ali moja sestra želi jabuku.

Za večeru means for dinner.

The preposition za often means for, and here it is used with the accusative case. The noun večera becomes večeru because it is in the accusative singular.

So:

• večera = dinner, supper
• za večeru = for dinner

This is a very common Serbian pattern:

• za doručak = for breakfast
• za ručak = for lunch
• za večeru = for dinner

Because they are direct objects and they are feminine singular nouns ending in -a.

In Serbian, feminine nouns of this type usually change:

• banana → bananu
• jabuka → jabuku

That is the accusative singular, which is commonly used for the thing someone wants, sees, eats, buys, etc.

So in this sentence:

• želim bananu = I want a banana
• želi jabuku = she wants an apple

Because čaj is a masculine inanimate noun.

In Serbian, masculine inanimate nouns usually have the same form in nominative and accusative singular.

So:

• čaj = tea
• želim čaj = I want tea

Compare that with feminine nouns:

• banana → bananu
• jabuka → jabuku

So the difference is caused by gender and case pattern.

They are two different forms of the verb želeti = to want.

In the present tense:

• ja želim = I want
• ti želiš = you want
• on/ona/ono želi = he/she/it wants

So in the sentence:

• želim = I want
• želi = she wants

The verb changes depending on the subject, just like English changes want to wants, but Serbian does this more systematically.

Because Serbian often drops subject pronouns when they are not needed.

The ending of the verb already tells you who the subject is:

• želim clearly means I want
• želi clearly means he/she/it wants

So ja želim is possible, but usually želim is enough.

Serbian includes pronouns like ja, ti, ona mostly for:

• emphasis
• contrast
• clarity

For example:

• Ja želim bananu, a ona želi jabuku.
  This sounds more contrastive: I want a banana, but she wants an apple.

Moja sestra means my sister.

The word moja is a possessive adjective meaning my, and it agrees with sestra in:

• gender
• number
• case

Since sestra is:

• feminine
• singular
• nominative

the possessive also appears as moja.

You could say just sestra if the context already made it clear whose sister you mean, but moja sestra is the normal way to say my sister.

Because Serbian adjectives and possessive words must agree with the noun they describe.

Here:

• sestra is feminine singular nominative
• so the possessive must also be feminine singular nominative
• that form is moja

A few examples:

• moj brat = my brother
• moja sestra = my sister
• moje dete = my child

So moja is not random; it matches the noun grammatically.

Because Serbian does not have articles like English a/an and the.

So:

• bananu can mean a banana or the banana
• čaj can mean tea, a tea, or the tea
• jabuku can mean an apple or the apple

The exact meaning depends on context.

This is very normal in Serbian, and learners have to get used to understanding definiteness from the situation rather than from an article.

Ali means but and shows a clear contrast.

So:

• Za večeru želim bananu i čaj, ali moja sestra želi jabuku. means:
• For dinner I want a banana and tea, but my sister wants an apple.

A learner may also see a, which can sometimes be translated as and or but depending on context. But ali usually sounds like a stronger, clearer contrast than a.

So here ali is very natural because the speaker is contrasting what I want with what my sister wants.

Serbian word order is more flexible than English word order, because case endings show grammatical roles.

The given sentence has a very natural neutral order:

• Za večeru želim bananu i čaj, ali moja sestra želi jabuku.

But other orders are also possible, especially for emphasis. For example:

• Bananu i čaj želim za večeru, ali moja sestra želi jabuku.
• Za večeru moja sestra želi jabuku, a ja želim bananu i čaj.

However, not every rearrangement sounds equally natural in every context. The original version is a good basic pattern for learners.

You can tell from both verb agreement and case forms.

• moja sestra želi = my sister wants
  Here sestra is the subject because the verb želi matches a third-person singular subject.

• bananu, čaj, and jabuku are things being wanted, so they are objects.

Their forms also help:

• bananu and jabuku are accusative forms
• čaj is masculine inanimate, so nominative and accusative look the same

So Serbian often uses endings, not just word order, to show who is doing the action and what receives the action.