Quan cau una fulla de la branca, l'ocell la mira una estona.

Quan means when.

Here it introduces a time clause: Quan cau una fulla de la branca = When a leaf falls from the branch.

Catalan uses quan very much like English when in sentences like this.

Cau is the 3rd person singular present tense of caure (to fall).

So:

• caure = to fall
• cau = it falls / falls

In this sentence, the subject is una fulla (a leaf), which is singular, so cau is the correct form.

Catalan often uses the present tense for:

• general truths
• habitual actions
• vivid narration

So Quan cau una fulla..., l'ocell la mira... can mean something like:

• When a leaf falls..., the bird watches it for a while
• Whenever a leaf falls..., the bird watches it for a while

It describes a typical event, not necessarily one specific past event.

Una fulla means a leaf, not the leaf.

The indefinite article una is used because the sentence is introducing a leaf in a general or non-specific way. If it said la fulla, it would sound like a particular leaf already known from the context.

De la branca means from the branch or literally of/from the branch.

Here de marks origin or source: the leaf falls from the branch.

Also note:

• de + la = de la
  There is no contraction here.

Compare that with forms like:

• de + el = del

So if the noun were masculine singular with el, you would usually get a contraction.

Because ocell begins with a vowel sound, the article el is reduced to l':

• el ocell → l'ocell

This apostrophe is very common in Catalan with articles before vowels or mute h.

So:

• l'ocell = the bird

La is a direct object pronoun meaning it, and it refers to una fulla (the leaf).

Since fulla is a feminine singular noun, the pronoun is la.

So:

• una fulla → later referred to as la
• l'ocell la mira = the bird watches it

In Catalan, object pronouns normally come before the conjugated verb.

So:

• la mira = watches it

This is different from English, where the object usually comes after the verb:

• English: the bird watches it
• Catalan: l'ocell la mira

This pronoun placement is one of the most important word-order differences for English speakers.

Yes, grammatically that is possible.

• L'ocell mira la fulla = The bird watches the leaf
• L'ocell la mira = The bird watches it

The version in your sentence uses the pronoun la to avoid repeating fulla. That is very natural in Catalan.

Here mirar means to look at, to watch, or to observe, depending on context.

In this sentence, the bird watches it for a while or looks at it for a while both fit well.

Catalan mirar is broader than English watch in some contexts, so the best translation depends on the situation.

Because mirar is a direct transitive verb in Catalan.

You simply say:

• mirar una cosa = to look at something

So Catalan does not need an extra preposition here, even though English often uses look at.

That is why the sentence has:

• l'ocell la mira

not something like an English-style literal equivalent with at.

Una estona means a while.

So:

• la mira una estona = it watches it for a while / it looks at it for a while

This is a very common Catalan expression for duration.

Catalan often expresses duration without a preposition where English uses for.

So:

• una estona = for a while
• dues hores = for two hours, in the right context

This is normal Catalan structure, not a missing word.

The comma separates the opening time clause from the main clause:

• Quan cau una fulla de la branca,
• l'ocell la mira una estona.

This is similar to English punctuation after an introductory clause:

• When a leaf falls from the branch, the bird watches it for a while.

Yes. Catalan often omits subject pronouns like he, she, or it when the verb form already shows the person and number.

In this sentence, the subjects are expressed with nouns:

• una fulla for cau
• l'ocell for mira

So there is no need for extra pronouns.

Yes, very often.

In a sentence like this, quan can suggest a repeated or typical situation:

• Whenever a leaf falls from the branch, the bird watches it for a while

That is why the present tense feels natural here.

Yes.

• branca = branch
• arbre = tree

So de la branca is specifically from the branch, not from the tree in general.

A fuller version would be:

Quan cau una fulla de la branca, l'ocell mira la fulla una estona.

That is understandable, but the original version is more natural because Catalan usually avoids unnecessary repetition:

Quan cau una fulla de la branca, l'ocell la mira una estona.