Mi hermana llenó la cantimplora otra vez cuando llegamos al arroyo.

Llenó is the preterite, which is used for a completed action in the past.

So mi hermana llenó la cantimplora means your sister filled the canteen at that moment.

If you said llenaba, that would suggest something more like:

• she was filling it
• she used to fill it
• the action was ongoing/background rather than a single completed event

Here, the sentence tells a clear past event, so llenó fits best.

For many -ar verbs, the present and preterite forms for nosotros look exactly the same.

So llegamos can mean either:

• we arrive
• we arrived

You work it out from the context. In this sentence, the rest is clearly in the past:

• llenó = she filled
• the whole sentence narrates a past situation

So cuando llegamos al arroyo is understood as when we arrived at the stream.

Because cuando is introducing the moment when something happened in the past.

In this sentence, both actions are treated as completed past events:

• llegamos = we arrived
• llenó = she filled

So cuando llegamos al arroyo means when we arrived at the stream.

This is very common in narration. If the sentence were describing a repeated or background situation, you might see the imperfect instead.

Because al is the mandatory contraction of a + el.

So:

• a + el arroyo → al arroyo

This happens only with el. It does not happen with la:

• a la playa
• a la montaña

Also, the verb llegar normally uses a before a destination:

• llegar al arroyo
• llegar al pueblo
• llegar a casa

Spanish often uses the definite article where English would use a possessive like his, her, or their, especially when the owner is already clear from context.

So la cantimplora here means the canteen / the water bottle, understood as the relevant one in the situation.

Using su cantimplora is possible, but it would put more focus on whose canteen it was. In this sentence, that detail is not the main point.

In Spain, cantimplora usually means a canteen or an outdoor water bottle, especially one used for hiking, camping, school trips, and similar situations.

It feels more specific than botella, which is just a general bottle.

So cantimplora suggests something like:

• a reusable bottle
• a hiking canteen
• a flask-style water container

Otra vez means again or once more.

So llenó la cantimplora otra vez means she filled the canteen again.

A common alternative is de nuevo, which also means again:

• Mi hermana llenó la cantimplora de nuevo

Another very common structure is:

• Mi hermana volvió a llenar la cantimplora

That also means my sister filled the canteen again.

That placement is very natural in Spanish. It modifies the action of filling and comes after the object.

So:

• llenó la cantimplora otra vez

is a normal word order.

Spanish word order is fairly flexible, but not every position sounds equally natural. For example:

• Mi hermana otra vez llenó la cantimplora is possible, but more marked
• Otra vez, mi hermana llenó la cantimplora emphasizes again

The original sentence sounds neutral and natural.

Spanish often omits subject pronouns because the verb ending usually tells you who the subject is.

Here:

• llenó tells you it is he/she/it
• mi hermana makes it clear that it is she
• llegamos tells you we

So adding ella or nosotros would usually be unnecessary unless you wanted emphasis or contrast.

For example:

• Ella llenó la cantimplora, no yo
• Nosotros llegamos primero

Yes. That is completely natural:

• Cuando llegamos al arroyo, mi hermana llenó la cantimplora otra vez.

This has the same basic meaning. The difference is mostly about information flow:

• starting with Mi hermana... puts the focus first on your sister
• starting with Cuando llegamos al arroyo... sets the scene first

Both are good Spanish.

The accent mark shows both stress and verb form.

• llenó = he/she filled → preterite, third person singular
• lleno = I fill or full depending on context

So the accent helps distinguish forms that would otherwise look very similar.

In llenó, the stress falls on the last syllable:

• lle-NÓ

Because arrival is being treated as a completed event.

Llegamos means we arrived: a finished moment in time.

That works well with the sentence because your arrival at the stream is the point when your sister filled the canteen again.

If you used llegábamos, it would mean something more like:

• we were arriving
• we used to arrive

That would change the feel of the sentence and make it less like a simple sequence of completed past events.

Not exactly.

Arroyo usually means a stream, brook, or small watercourse, while río means a river.

So arroyo suggests something smaller than a river. In Spain, it can also sometimes refer to a stream that may not always carry much water, depending on the season.

So the choice of arroyo gives a more specific picture of the place.