Unu amiko bakas panon, kaj alia gustumas la saŭcon.

Here unu literally means one, but in this kind of pattern it works like one ... another.

So unu amiko ... kaj alia ... means something like:

• one friend ... and another ...

It is not just an indefinite a friend. It sets up a contrast between two different people.

If you removed unu, the sentence would feel less balanced. The pair unu ... alia is a very natural way to show one ... another.

Because the noun is understood from the previous part of the sentence.

• unu amiko = one friend
• alia = another friend

In Esperanto, an adjective can stand alone when the noun is obvious from context. So alia here really means alia amiko, but repeating amiko is unnecessary.

Because it is still grammatically an adjective, even when the noun is omitted.

Esperanto adjectives always end in -a. So:

• alia amiko = another friend
• alia = another one / the other one

The missing noun is understood, but the adjective keeps its normal adjective ending.

They are direct objects, so they take the accusative ending -n.

In this sentence:

• bakas panon = bakes bread
• gustumas la saŭcon = tastes the sauce

The -n shows what is being baked and what is being tasted.

This is one of the most important patterns in Esperanto: the thing directly affected by the verb usually gets -n.

Because they are the subjects, not the objects.

• Unu amiko bakas ... → the friend is doing the baking
• alia gustumas ... → the other friend is doing the tasting

Subjects normally do not take -n. The accusative -n is for direct objects, direction in some cases, and a few other special uses.

Because la marks something definite or specific.

So the sentence suggests:

• panon = bread in a general or indefinite sense
• la saŭcon = the sauce, a particular sauce already known in the situation

Esperanto has only one article, la, which corresponds to English the. There is no separate word for a/an. When something is indefinite, Esperanto often simply uses no article.

-as marks the present tense.

So:

• bakas = is baking / bakes
• gustumas = is tasting / tastes

Esperanto tense endings are very regular:

• -as = present
• -is = past
• -os = future

So if you changed the tense, you would get:

• bakis = baked
• bakos = will bake

Not strictly.

Esperanto punctuation is somewhat flexible here. Many writers would write:

• Unu amiko bakas panon kaj alia gustumas la saŭcon.

Others include the comma because there are two full clauses:

• Unu amiko bakas panon, kaj alia gustumas la saŭcon.

So the comma is acceptable, but not required.

A few useful pronunciation points:

• c is always pronounced like ts
• g is always a hard g, as in go
• u sounds like oo in food
• aŭ is a diphthong, roughly like the ow in cow
• stress is always on the second-to-last syllable

So:

• saŭcon ≈ SOW-tson
  Stress on saŭ
• gustumas ≈ goos-TOO-mas
  Stress on tu

Usually, yes, that is the most natural reading.

It presents one person and then another person in contrast:

• one friend does one action
• another friend does the other action

So most readers will understand that two different friends are involved.

If you wanted, you could also say alia amiko explicitly, but alia by itself already makes that clear.

Not exactly.

gustumi specifically means to taste something with the mouth, especially food or drink, often to check the flavor.

provi usually means to try or test something more generally.

So here:

• gustumas la saŭcon = tastes the sauce

That is more precise than provas la saŭcon, because it clearly refers to tasting food rather than just trying something in a broader sense.