Aldonu la saŭcon.

Because -u is the Esperanto verb ending for a command, request, or exhortation.

So:

• aldoni = to add
• aldonu = add! / please add / let someone add

In this sentence, Aldonu la saŭcon. means Add the sauce.

Unlike English, Esperanto uses the same -u form whether you are talking to one person or more than one person. Context tells you who is being addressed.

The dictionary form is aldoni, meaning to add.

You can break it down like this:

• al- = to, toward
• don- = give
• -i = infinitive ending

Historically and structurally, aldoni gives the idea of adding on or giving to something.

From that verb you get forms like:

• mi aldonas = I add / am adding
• mi aldonis = I added
• mi aldonos = I will add
• aldonu = add!

The -n marks the direct object.

In Aldonu la saŭcon:

• Aldonu = add
• la saŭcon = the sauce

The thing being added is the direct object, so saŭco becomes saŭcon.

This is one of the most important features of Esperanto grammar:

• nouns normally end in -o
• direct objects add -n

So:

• saŭco = sauce
• saŭcon = sauce, as the object of the verb

La is the definite article, meaning the.

So:

• saŭcon = sauce
• la saŭcon = the sauce

You use la when you mean a specific sauce already known from the context.

For example:

• Aldonu saŭcon. = Add some sauce / add sauce.
• Aldonu la saŭcon. = Add the sauce.

Esperanto has only one article, la, and it does not change for gender, number, or case.

A good approximate pronunciation is SOW-tson.

More exactly:

• saŭ sounds like English sow or ow in cow
• c in Esperanto is always ts
• o is a pure o
• the final -n is pronounced clearly

So saŭcon is roughly:

SAŬ-tson

Also remember that Esperanto stress normally falls on the second-to-last syllable, so here the stress is on saŭ: SAŬ-con

In Esperanto, ŭ is a short w-like sound used mainly in diphthongs, especially aŭ and eŭ.

In saŭco, the combination aŭ is pronounced as one unit, similar to the sound in English cow.

So saŭco is not pronounced like two separate vowels sa-u-co. Instead, aŭ blends together:

• saŭ-co

This is a common point for English speakers, because ŭ is not a normal English letter.

It can be either, depending on tone and context.

The -u form in Esperanto covers:

• commands
• requests
• instructions
• polite directions

So Aldonu la saŭcon can mean:

• Add the sauce.
• Please add the sauce.

If you want to make it explicitly more polite, you can add words such as:

• Bonvolu aldoni la saŭcon. = Please add the sauce.
• Mi petas, aldonu la saŭcon. = Please, add the sauce.

So the bare -u form is not automatically rude.

Yes, word order in Esperanto is fairly flexible, because the -n ending helps show what the object is.

The normal, neutral order is:

• Aldonu la saŭcon.

But other orders are possible for emphasis, such as:

• La saŭcon aldonu.

That sounds more marked or emphatic, something like The sauce, add it.

For learners, it is best to use the normal order first: verb + object here.

No, not the verb form itself.

Aldonu works for:

• one person
• several people

Esperanto does not change the imperative form for number the way English sometimes distinguishes add versus you all add only by context.

If needed, you can make the subject explicit:

• Vi aldonu la saŭcon. = You should add the sauce.
• Vi ĉiuj aldonu la saŭcon. = You all add the sauce.

But normally Aldonu la saŭcon is enough.

It is a noun.

In Esperanto, nouns end in -o, so:

• saŭco = sauce

Then:

• saŭcon = sauce as a direct object
• saŭcoj = sauces
• saŭcojn = sauces as direct objects

This regular system is very typical of Esperanto and makes it easier to recognize word classes.

Yes. Saŭco is a normal Esperanto word.

Esperanto includes many roots that come from European languages, but they are adapted to Esperanto spelling and sound patterns. Here:

• English sauce
• Esperanto saŭco

Notice how it is made to fit Esperanto rules:

• noun ending -o
• regular pronunciation
• regular grammar: saŭco, saŭcon, saŭcoj, saŭcojn

So even if the word looks familiar, it behaves completely regularly in Esperanto.