Donu po du porciojn da pasto al ĉiu gasto.

Donu is the -u verb form in Esperanto. This form is used for commands, requests, wishes, and similar ideas.

So here donu means something like:

• give
• please give
• let someone give, depending on context

In this sentence, it is most naturally a command or instruction to you.

In Esperanto, commands often leave out the subject when it is understood. So:

• Donu ... = Give ...
• Vi donu ... = You give ...

Just like in English, the subject is usually omitted in commands. The -u ending already signals that this is a command/request form.

Po shows distribution. It means something like:

• each
• apiece
• at the rate of

So po du porciojn means two portions each.

A useful way to think of it is:

• du porciojn = two portions
• po du porciojn = two portions for each one in the relevant group

They look similar, but they do different jobs.

• po = distributive, meaning each/apiece
• por = for, showing purpose, destination, or intended recipient

Examples:

• po du pomoj = two apples each
• du pomoj por la infanoj = two apples for the children

In your sentence, po tells us how the portions are distributed, while al ĉiu gasto tells us who receives them.

Because po tells you the pattern of distribution, but not by itself exactly who gets the items.

• po du porciojn = in groups of two each
• al ĉiu gasto = to each guest

So together they mean that every guest receives two portions.

You can think of it this way:

• po answers: how many per person?
• al ĉiu gasto answers: per whom?

It has two endings because it is:

• plural: -j
• accusative: -n

So:

• porcio = one portion
• porcioj = portions
• porciojn = portions as a direct object

It is plural because there are two portions, and accusative because the portions are what is being given.

Because ordinary cardinal numbers in Esperanto usually do not take -j or -n.

So you say:

• du porcioj
• du porciojn

not forms like duj or dun.

The noun shows the grammatical ending, not the number word.

Da is used after words showing quantity, measure, or amount, especially with things thought of as substance or mass rather than separate countable items.

Here:

• du porcioj da pasto = two portions of pasta

The word porcioj gives the countable units, and da pasto tells what those portions consist of.

By contrast, de usually has other meanings, such as possession, origin, or a more specific relation. After a quantity expression like porcioj, da is the normal choice.

Because al marks the recipient, the person something is given to.

So:

• doni ion al iu = to give something to someone

In this sentence:

• po du porciojn da pasto = the thing being given
• al ĉiu gasto = the recipient

This is very common with doni and similar verbs.

Because ĉiu means each or every, and it normally goes with a singular noun:

• ĉiu gasto = each guest

If you said ĉiuj gastoj, that would mean all the guests as a group.

Both can refer to the same people, but the viewpoint is different:

• ĉiu gasto = one by one, individually
• ĉiuj gastoj = all of them together

Since the sentence is about giving a certain amount to each individual person, ĉiu gasto fits perfectly.

Because gasto is not the direct object here.

The direct object is porciojn. The guests are the recipients, and recipients are marked with al, not with the accusative -n.

So:

• Donu porciojn = Give portions
• al ĉiu gasto = to each guest

Once al is there, you normally do not add accusative to the noun.

Yes. Esperanto word order is fairly flexible because endings and prepositions show the grammatical roles.

For example, these are all possible:

• Donu po du porciojn da pasto al ĉiu gasto.
• Al ĉiu gasto donu po du porciojn da pasto.
• Po du porciojn da pasto donu al ĉiu gasto.

They all mean basically the same thing, though the emphasis may shift slightly. The original version is a natural neutral order.

A simple breakdown is:

• Donu = Give
• po du porciojn = two portions each
• da pasto = of pasta
• al ĉiu gasto = to each guest

So the structure is:

verb + direct object + recipient

That is a very common pattern in Esperanto with verbs like doni.