Learn Hardcore Esperanto: Unu birdo flugas sur la arbon, kaj alia jam sidas sur la arbo. - One bird flies onto the tree, and another already sits on the tree.

birdo flugas sur la arbon – a bird is flying onto the tree (indefinite, just “some bird”).
unu birdo flugas sur la arbon – one bird (or one particular bird) is flying onto the tree, often implying contrast with other birds (for example, with the alia in the second clause).

So unu is the numeral “one”, and it slightly emphasizes the individuality of that bird, or helps set up the contrast unu … alia = one … another.

Why is it “unu birdo” (singular) instead of “unu birdoj” (plural)?

In Esperanto, numerals don’t force plural endings when the number is one:

unu birdo = one bird (singular noun)
du birdoj = two birds
tri birdoj = three birds

So unu goes with a singular noun, which is why it’s birdo, not birdoj.

Why is there no -n on birdo in “unu birdo flugas …”?

The -n ending marks the accusative, mainly used for:

direct objects, and
showing direction with some prepositions.

Here, unu birdo is the subject, not an object, and subjects stay in the nominative, which has no -n. So:

Subject: unu birdo
Object: mi vidas birdon – I see a bird (here birdon is the object).

Why is it “sur la arbon” in the first clause but “sur la arbo” in the second?

This is the key contrast:

sur la arbon – onto the tree (movement towards the top of the tree)
sur la arbo – on the tree (location, no movement)

In Esperanto, some prepositions (including sur) can take:

nominative (no -n) → place where something is
- lia ĉapelo estas sur la tablo – His hat is on the table.
accusative (-n) → direction / movement to a place
- li metas la ĉapelon sur la tablon – He puts the hat onto the table.

So:

flugas sur la arbon – is flying onto the tree (direction).
sidas sur la arbo – is sitting on the tree (static location).

Could we say “al la arbo” instead of “sur la arbon”?

Yes, but the meaning changes slightly:

flugas al la arbo – flies to(ward) the tree (destination is the tree in general; maybe it will land on it, in it, or near it, we’re not specifying).
flugas sur la arbon – flies onto the tree (specifically onto its surface, like onto a branch).

So al la arbo is more general: movement to the tree.
sur la arbon is more specific: movement onto the tree (onto its surface).

Why is it “alia jam sidas” instead of repeating “alia birdo jam sidas”?

Esperanto often omits a noun when it is obvious from context. Alia can work like:

an adjective: alia birdo – another bird
a pronoun: alia – another (one)

In this sentence, birdo was just mentioned, so alia clearly means alia birdo = another bird. Repeating birdo is not wrong; it’s just unnecessary:

Unu birdo flugas sur la arbon, kaj alia birdo jam sidas sur la arbo. – grammatically fine, just a bit heavier.

Why is it “alia” and not “la alia”?

Both are possible, but there’s a nuance:

alia = another (one), a different one – indefinite.
la alia = the other (one) – more definite, implying “the specific other one we have in mind”.

In a context where you clearly have two birds, you might say:

Unu birdo flugas sur la arbon, kaj la alia jam sidas sur la arbo.
→ One bird is flying onto the tree, and the other is already sitting on the tree.

With just alia, it sounds slightly more like one bird … and another (one), without strongly stressing that there are exactly two.

Why does “alia” not have an -n or -j ending here?

Alia is:

singular (refers to one other bird),
the subject of sidas in its clause.

So it’s nominative singular → alia (no -n, no -j).

If it were plural or in another role, it would change:

Plural: aliaj birdoj sidas sur la arbo – other birds are sitting on the tree.
Object: mi vidas alian birdon – I see another bird.
Plural object: mi vidas aliajn birdojn – I see other birds.

Why do we use “la” in “la arbon / la arbo”? Could we say it without la?

La is the definite article (“the”). Using la arbo means:

the tree – a specific tree already known in the context.

If you say arbo without la, it becomes indefinite:

unu birdo flugas sur arbon, kaj alia jam sidas sur arbo – literally: one bird is flying onto a tree, and another already sits on a tree.
That would sound like two possibly different, unspecified trees.

With la arbo, you indicate it’s the same specific tree in both clauses.

Why is it “sur la arbo” and not “en la arbo”? In English we say “in a tree” for birds.

Both sur la arbo and en la arbo are grammatically correct, but they have different images:

sur la arbo – on the tree, i.e., on its surface: on a branch, on the trunk, etc.
en la arbo – in the tree, i.e., within the space formed by the tree, among the branches, foliage, etc.

In many contexts with birds, Esperanto speakers actually use en la arbo, because the bird is in among the branches:

La birdo sidas en la arbo – The bird is sitting in the tree.

Your sentence uses sur, which suggests the bird is sitting on the tree (on its surface), e.g. on a branch. It’s not wrong; it just paints a slightly different picture.

Does “flugas” here mean “is flying” or “flies”? How does tense and aspect work?

The verb ending -as marks present tense, but not a specific aspect (continuous vs simple). So:

flugas can correspond to both:
- is flying (present continuous),
- flies (present simple).

Context decides which English form feels more natural. Here, because we talk about an event happening now while contrasting with another bird already sitting, English speakers will usually read it as:

One bird is flying onto the tree, and another is already sitting on the tree.

Could we change the word order, for example “Unu birdo sur la arbon flugas”?

Yes, Esperanto has relatively flexible word order. You can say:

Unu birdo flugas sur la arbon. (neutral)
Unu birdo sur la arbon flugas.
Sur la arbon flugas unu birdo.

All are grammatically correct. The changes mainly affect:

rhythm,
emphasis (e.g. putting sur la arbon first can emphasize the destination).

However, for beginners and for maximum clarity, the neutral Subject–Verb–(other elements) order, as in the original sentence, is usually preferred.

Could we replace “la arbo” in the second clause with “ĝi”?

Yes, that’s possible:

Unu birdo flugas sur la arbon, kaj alia jam sidas sur ĝi.

Here, ĝi refers back to la arbo (an inanimate singular noun), and that’s normal in Esperanto: ĝi = it. This also avoids repeating la arbo if the context is clear.

Both versions are correct; using ĝi just makes the sentence slightly less repetitive.