Gustumu la saŭcon.

The ending -u is the Esperanto volitive ending. It is used for commands, requests, invitations, and similar ideas.

So gustumu means something like:

• Taste!
• Please taste.
• Do taste.

In this sentence, it is a command or invitation directed at the listener.

Yes. In a sentence like Gustumu la saŭcon, the subject is usually understood as vi.

So the full idea is:

• (Vi) gustumu la saŭcon.

Because Esperanto often leaves out vi in commands, the sentence sounds natural without it.

Also, vi can mean you singular or plural, so this could be said to one person or several people.

The -n marks the direct object.

Here:

• gustumu = taste
• la saŭcon = the sauce

The sauce is the thing being tasted, so it takes the accusative ending -n.

Compare:

• la saŭco = the sauce
• la saŭcon = the sauce as a direct object

This is a very common Esperanto pattern.

La is the definite article, meaning the.

So:

• saŭco = sauce
• la saŭco = the sauce

Esperanto has only one definite article: la. It does not change for gender, number, or case. Even when the noun gets -n, la stays the same:

• la saŭco
• la saŭcon

A helpful approximation is SAHW-tson.

A few pronunciation points:

• aŭ sounds like a diphthong similar to ow in cow
• c in Esperanto is always ts
• the final -n is pronounced clearly

So saŭco sounds roughly like SAHW-tso, and saŭcon adds the final n.

A rough English approximation is goos-TOO-moo.

More exactly:

• g is always a hard g, as in go
• u sounds like oo in food
• stress falls on the second-to-last syllable

So the stress is on tu:

• gus-TU-mu

This is the most straightforward word order for a command:

• verb first
• object after it

So:

• Gustumu la saŭcon = Taste the sauce

Because Esperanto marks the object with -n, word order is fairly flexible, but this version is the most neutral and natural for a simple command.

It can be either, depending on context and tone.

In Esperanto, the -u form covers a range that in English might be expressed as:

• Taste the sauce.
• Please taste the sauce.
• Do try the sauce.

So Gustumu la saŭcon could sound like an order, but it could also be a polite invitation, especially in a context like offering food.

Yes, that is possible.

• Gustumu la saŭcon = normal, concise command
• Vi gustumu la saŭcon = adds emphasis to you

Including vi can make the subject more explicit or contrastive, as if saying:

• You taste the sauce.

But in most ordinary situations, leaving out vi is more natural.

It is a normal Esperanto word, but like many Esperanto words, it comes from international vocabulary. Saŭco is clearly related to words like sauce in English and similar forms in several European languages.

Its structure is fully Esperanto, though:

• saŭc-o = sauce
• saŭc-on = sauce as a direct object

So even when a word is internationally familiar, it still follows regular Esperanto grammar.