Trebalo bi da poslužimo hranu i piće.

Trebalo bi is a very common Serbian way to express should, ought to, or it would be good to.

In this sentence, it gives a sense of recommendation or mild obligation, not a strong command.

A useful way to think of it is:

• treba = it is necessary / need to
• trebalo bi = should / ought to

So Trebalo bi da poslužimo... means something like We should serve...

These forms are related, but they do not feel exactly the same.

• trebamo da poslužimo hranu i piće = we need to serve food and drink / we have to serve...
• trebalo bi da poslužimo hranu i piće = we should serve food and drink

So trebalo bi is softer and more like advice, suggestion, or the appropriate thing to do.

Also, trebalo here is used in an impersonal construction. Serbian often uses this pattern:

• trebalo bi da + present tense

That pattern is extremely common for should.

In Serbian, a clause with da + present tense often does the job that an infinitive does in English.

So instead of saying something structurally like should serve, Serbian very naturally says:

• trebalo bi da poslužimo

literally closer to it would be needed that we serve

This sounds normal and idiomatic in Serbian.

You may also sometimes see an infinitive after similar verbs, but da + present is one of the most important and frequent patterns in modern Serbian.

Poslužimo is the 1st person plural present tense form of poslužiti.

That means:

• mi poslužimo = we serve

After da, Serbian normally uses a present-tense form:

• da poslužim = that I serve
• da poslužiš = that you serve
• da poslužimo = that we serve

So in this sentence, poslužimo is there because the meaning is that we serve.

This is mainly about aspect.

• poslužiti is perfective
• služiti is usually imperfective

In this sentence, poslužiti suggests a complete, single action: serving the food and drink as a task to be done.

So:

• Trebalo bi da poslužimo hranu i piće. = We should serve the food and drink.
  This sounds like a concrete action to carry out.

If you used služimo, it would more likely suggest an ongoing or repeated activity, which does not fit this context as well.

They are in the accusative, because they are the direct objects of the verb poslužimo.

The verb answers serve what?

• hrana → hranu
• piće → piće

So:

• hranu is accusative singular of hrana
• piće is neuter singular, and its nominative and accusative forms are the same

That is why one word changes and the other does not.

Because hrana is a feminine noun, and in the singular accusative it changes its ending from -a to -u.

So:

• nominative: hrana
• accusative: hranu

This is a very common pattern for feminine nouns ending in -a.

Compare:

• voda → vodu
• kafa → kafu
• salata → salatu

Piće is a neuter noun, and in the singular its nominative and accusative are the same.

So:

• nominative: piće
• accusative: piće

This is normal for many neuter nouns in Serbian.

For example:

• mleko → mleko
• vino → vino
• piće → piće

Grammatically, piće is singular.

However, like English drink or beverage, it can have a general or mass-noun meaning. So hranu i piće often means food and drink in a broad sense, not necessarily one single drink.

Depending on context, English might translate it as:

• food and drink
• food and beverages
• food and drinks

But in Serbian, piće here is singular in form.

Because Serbian often leaves out subject pronouns when they are already clear from the verb ending.

The ending -mo in poslužimo already tells you the subject is we.

So:

• (mi) poslužimo = we serve

The pronoun mi can be added for emphasis, contrast, or clarity, but it is not necessary.

So both are possible:

• Trebalo bi da poslužimo hranu i piće.
• Trebalo bi da mi poslužimo hranu i piće.

The second one puts more emphasis on we.

Bi is a particle used in the conditional.

In trebalo bi, it helps create the softer meaning should / ought to, rather than a plain statement of necessity.

So compare:

• Treba da poslužimo hranu i piće. = We need to / have to serve food and drink.
• Trebalo bi da poslužimo hranu i piće. = We should serve food and drink.

Also, bi is a clitic, which means it usually appears in a special position near the beginning of the clause. That is why it comes right after trebalo.

Yes. Serbian word order is fairly flexible, although some versions sound more natural in certain contexts.

The neutral version is:

• Trebalo bi da poslužimo hranu i piće.

But you could also say:

• Hranu i piće bi trebalo da poslužimo.
• Trebalo bi hranu i piće da poslužimo.

These alternatives may shift emphasis a little:

• putting hranu i piće earlier can emphasize what should be served
• keeping the original order sounds most neutral and straightforward

So the original sentence is a very natural default word order.

Yes, you may also encounter:

• Trebalo bi poslužiti hranu i piće.

This is also understandable and grammatical.

The difference is that:

• trebalo bi da poslužimo explicitly includes we
• trebalo bi poslužiti is a bit more impersonal, closer to food and drink should be served or one should serve food and drink

In everyday Serbian, da + present is extremely common and often more conversational.

Trebalo is the neuter singular form of the past active participle of trebati, but in this kind of sentence it functions as part of an impersonal expression.

So learners do not need to think of it as a normal past-tense verb meaning needed here. In this pattern:

• trebalo bi da...

it simply forms the modal meaning should.

This is one of those cases where the whole expression is more important than the individual literal pieces. It is best learned as a set pattern:

• trebalo bi da + present tense = should / ought to