Yo abrí el saco de dormir y mi hija se tumbó enseguida, porque estaba cansadísima.

Yes — Spanish often leaves out subject pronouns because the verb ending already tells you who is doing the action. So Abrí el saco de dormir... would already mean I opened the sleeping bag...

Here, yo is optional. It may be included for:

• emphasis: I opened it
• contrast: I opened the sleeping bag and my daughter lay down
• clarity in context

So Yo abrí... and Abrí... are both correct.

Abrí is the preterite, which is used for a completed action in the past. Opening the sleeping bag is seen as a single finished event.

• abrí = I opened
• abría = I was opening / I used to open

In this sentence, the speaker is narrating a sequence of events:

1. I opened the sleeping bag.
2. My daughter lay down.
3. She was tired.

That is why abrí fits better than abría.

El saco de dormir is the normal Spanish way to say sleeping bag.

Literally, it is:

• saco = bag / sack
• de = for / of
• dormir = to sleep

So it is basically bag for sleeping.

Spanish often uses this structure:

• gafas de sol = sunglasses
• cepillo de dientes = toothbrush
• mesa de comedor = dining table

So saco de dormir is a very natural noun phrase in Spanish.

Because tumbarse is the common verb meaning to lie down.

Compare:

• tumbar algo = to knock something down / lay something flat
• tumbarse = to lie down oneself

So:

• Mi hija tumbó la silla = My daughter knocked over the chair
• Mi hija se tumbó = My daughter lay down

The se is part of the verb here and shows that the subject is putting herself into that position.

Se tumbó is preterite, which presents the action as a completed event: she lay down.

Se tumbaba would suggest:

• an ongoing action in the past
• a habitual action
• background description

In this sentence, se tumbó enseguida means she lay down right away as one clear event, so the preterite is the natural choice.

Enseguida usually means:

• right away
• immediately
• straight away

It is a very common adverb in Spanish.

Examples:

• Voy enseguida. = I’m coming right away.
• Se durmió enseguida. = He/She fell asleep immediately.

In some contexts, especially depending on region or tone, it can also mean very soon, but here it clearly means right away.

Because porque is the conjunction meaning because.

Spanish distinguishes these forms:

• porque = because
• por qué = why
• porqué = the reason / the why
• por que = less common combination in special structures

Here the sentence gives a reason:

• ...porque estaba cansadísima. = ...because she was very tired.

So porque is correct.

Estaba is the imperfect, and here it gives background information or a state: she was very tired.

In past narration, Spanish often uses:

• preterite for main completed events
• imperfect for descriptions, conditions, feelings, or ongoing states

So the sentence works like this:

• abrí = completed event
• se tumbó = completed event
• estaba cansadísima = background state explaining why

Using estuvo would sound more like treating the tiredness itself as a bounded, completed event, which is less natural here.

Cansadísima means extremely tired / very, very tired.

It comes from:

• cansada = tired
• -ísima = a suffix that intensifies the adjective

So:

• cansada = tired
• muy cansada = very tired
• cansadísima = extremely / really / incredibly tired

This suffix is very common in Spanish:

• guapa → guapísima
• rico → riquísimo
• pequeño → pequeñísimo

It is often a bit more expressive than just using muy.

Because it describes mi hija, which is feminine singular.

Adjectives in Spanish usually agree with the noun they describe:

• cansado = masculine singular
• cansada = feminine singular
• cansadísimos = masculine plural
• cansadísimas = feminine plural

Since hija is feminine singular, cansadísima is the correct form.

It most naturally refers to mi hija.

Why? Because the closest and most logical subject is mi hija, and she is the one who lay down immediately, so the reason because she was extremely tired clearly matches her.

Also, cansadísima is feminine singular, which matches hija. If the speaker were a man, he would say cansadísimo about himself. If the speaker were a woman, cansadísima could in theory match yo, but in this sentence the natural interpretation is still the daughter.

Because possessives like mi, tu, su, nuestro, etc. normally come without an article.

So:

• mi hija = my daughter
• mi casa = my house
• nuestro coche = our car

You generally do not say la mi hija in standard modern Spanish.

Absolutely. In fact, Abrí el saco de dormir y mi hija se tumbó enseguida, porque estaba cansadísima may sound even more neutral in everyday Spanish.

Including yo is not wrong at all — it just adds a little emphasis or helps structure the sentence more clearly.

They are there for pronunciation and stress.

• abrí: the accent shows stress on the last syllable: a-BRÍ
• tumbó: stress on the last syllable: tum-BÓ
• cansadísima: the stress falls on -sí-: can-sa-DÍ-si-ma

In verbs like abrí and tumbó, the accents also help distinguish the correct past forms.

For example:

• abri would be incorrect
• tumbo means I knock down / I lie down in the present
• tumbó means he/she knocked down / lay down in the preterite

So the accents matter both for pronunciation and meaning.