Una ardilla bajó del árbol y se escondió detrás del banco.

Una introduces the squirrel as new information: it is a squirrel, not a specific one the listener already knows about.

You would use la ardilla if:

• the squirrel had already been mentioned
• both speaker and listener already knew which squirrel it was
• the context made it specific

So una ardilla is the natural way to begin this little scene.

No. Ardilla is a grammatically feminine noun, so it takes una, but that does not necessarily mean the actual animal is biologically female.

In Spanish, grammatical gender and real-world sex are not always the same thing. Here, una ardilla just means a squirrel.

Del is the contraction of de + el.

So:

• de el árbol becomes del árbol
• literally, from the tree

This contraction is required with the article el.

The same thing happens later in the sentence with del banco:

• detrás de el banco becomes detrás del banco

They are in the preterite because the sentence describes two completed actions in sequence:

• first, the squirrel came down
• then, it hid

This is exactly the kind of thing the preterite is used for in Spanish: finished events that move a story forward.

If you used the imperfect, such as bajaba or se escondía, it would sound more like background description, habit, or an action in progress rather than a completed event.

Bajó del árbol is the most neutral, straightforward way to say that it came down from the tree.

Bajarse is also possible in some contexts, but it is especially common when talking about:

• getting off a vehicle
• getting down from somewhere in a more self-directed or colloquial way

For an animal coming down from a tree, plain bajar is the simplest and most natural choice.

Because esconderse means to hide oneself / to hide.

Compare:

• escondió algo = it hid something
• se escondió = it hid itself / it hid

So the se is needed here because the squirrel is not hiding another object; it is the one doing the hiding.

Yes, in form it is reflexive or, more precisely, pronominal.

In practice, learners usually just need to remember:

• esconder = to hide something
• esconderse = to hide

So even though English usually just says hide, Spanish normally uses the pronominal form when the subject hides itself.

No. Detrás normally needs de before a noun.

So:

• detrás de + el banco = detrás del banco

You need the full prepositional structure:

• detrás del banco
• detrás de la puerta
• detrás de mí

So detrás banco would be incorrect.

It can mean either, depending on context.

Common meanings include:

• bench
• bank

In this sentence, bench is much more likely, because a squirrel hiding behind a bench makes sense in a park or outdoor scene. Hiding behind a financial institution would be a very odd image.

Spanish often leaves out subject pronouns when the subject is already clear.

Here, una ardilla has already been mentioned, so Spanish does not need to repeat:

• ella bajó
• ella se escondió

Just bajó and se escondió are enough.

This is very normal in Spanish and much more common than in English.

They show which syllable is stressed.

• árbol: stress on the first syllable
• bajó: stress on the last syllable
• escondió: stress on the last syllable

In bajó and escondió, the accent also helps distinguish these past-tense forms from other forms that would be pronounced differently.

So the accents are not optional spelling details; they are part of the correct written form.

In most of Spain, ardilla is pronounced roughly like ar-DI-ya, with the stress on di.

A few helpful points:

• ar as in a short Spanish ar
• di is the stressed syllable
• ll in most modern Spanish pronunciation sounds similar to the y in yes

So a rough English approximation would be ar-DEE-ya, though the real Spanish vowels are shorter and cleaner than in English.