Cuando me levanté, estaba tan mareada que tuve que sentarme en el sofá.

Because levantarse is a reflexive verb in this context: to get up / to stand up.

• levantar = to lift, to raise something
• levantarse = to get up

So:

• levanté la caja = I lifted the box
• me levanté = I got up

The me shows that the action happens to yourself.

This is a very common Spanish question: preterite vs imperfect.

• me levanté is preterite because it refers to a completed action: I got up
• estaba is imperfect because it describes a state or condition in the background: I was feeling / I was

So the sentence structure is:

• first, a completed event happened: Cuando me levanté = When I got up
• at that moment, there was a state: estaba tan mareada = I was so dizzy

Spanish often uses:

• preterite for actions/events
• imperfect for descriptions, conditions, ongoing states

Because here cuando means when I got up / once I got up, referring to a specific completed moment.

• Cuando me levanté = when I got up, at that specific time
• Cuando me levantaba would suggest something more habitual or ongoing, like when I was getting up or whenever I used to get up, depending on context

In this sentence, the speaker means one particular occasion, so levanté is the natural choice.

Because the adjective agrees with the speaker’s gender.

• mareado = dizzy, masculine
• mareada = dizzy, feminine

So if the speaker is a woman, she says:

• estaba mareada

If the speaker is a man, he would say:

• estaba mareado

This kind of adjective agreement is very common in Spanish.

Here mareada means dizzy.

Depending on context, mareado/a can also mean:

• light-headed
• nauseous
• motion sick

In this sentence, because she had to sit down, dizzy is the most natural interpretation.

Tan ... que means so ... that.

In the sentence:

• tan mareada que tuve que sentarme
• so dizzy that I had to sit down

This pattern is very common:

• tan cansado que me dormí = so tired that I fell asleep
• tan caro que no lo compré = so expensive that I didn’t buy it

So:

• tan = so
• que = that

Both can mean had to, but they are not used in exactly the same way.

• tuve que = I had to, for a specific occasion or completed necessity
• tenía que = I had to / I was supposed to / I needed to, often more ongoing or descriptive

Here the sentence refers to one specific moment:

• I got up
• I felt very dizzy
• I had to sit down

That makes tuve que the better choice.

Compare:

• Ayer tuve que sentarme = Yesterday I had to sit down
• Cuando estaba enferma, tenía que sentarme a menudo = When I was ill, I had to sit down often

Because after tener que, Spanish normally uses an infinitive.

• tuve que sentarme = I had to sit down

Not:

• tuve que me senté ❌

The structure is:

• tener que + infinitive

Examples:

• tuve que salir = I had to leave
• tenemos que estudiar = we have to study
• tuve que sentarme = I had to sit down

Since sentarse is reflexive, the reflexive pronoun stays attached to the infinitive here:

• sentarme = to sit myself down / to sit down

No. With a reflexive infinitive after another verb, the pronoun can either:

• attach to the infinitive, or
• go before a conjugated verb

But with tener que + infinitive, the normal form is:

• tuve que sentarme

You cannot place me between que and the infinitive:

• tuve que me sentar ❌

Compare with other structures:

• quiero sentarme
• me quiero sentar

Both are possible there.

But with tener que, the natural form is:

• tengo que sentarme

Because sentarse en is the usual way to say to sit on/in a seat, sofa, chair, etc.

• sentarse en el sofá = to sit on the sofa
• sentarse en la silla = to sit on the chair

Using a would suggest movement toward something, but Spanish normally uses en with places where you end up seated.

So:

• me senté en el sofá = I sat down on the sofa

is the standard phrasing.

Because Spanish often leaves out subject pronouns when the verb already shows who the subject is.

• me levanté clearly means I got up
• estaba in this context also refers to I was
• tuve clearly means I had

So yo is not necessary.

You could say yo for emphasis or contrast:

• Yo tuve que sentarme, pero ella no = I had to sit down, but she didn’t

But normally Spanish prefers to omit it.

Yes, it is basically the same idea: both verbs are reflexive here.

• me levanté = I got up
• sentarme = to sit down

In both cases, the action refers back to the subject.

For yo, the reflexive pronoun is me:

• me levanto
• me senté
• sentarme

For other people:

• te levantaste
• se sentó
• nos sentamos

So the reflexive pronoun changes with the subject.

Because Spanish commonly expresses physical states with estar + adjective.

• estaba mareada = I was dizzy

That is very natural.

You could also use marearse in other contexts, for example:

• me mareé al levantarme = I got dizzy when I got up

But in this sentence, the speaker is describing her condition at that moment, so estaba mareada works very well.

Yes, they are important.

• levanté has an accent to show the stress and to distinguish the correct written form of the yo preterite
• sofá has an accent because the stress falls on the last syllable

Accent marks in Spanish are not optional in standard writing. They can change pronunciation and sometimes meaning.

So write:

• levanté
• sofá

not:

• levante ❌ in this context
• sofa ❌

Yes, it sounds completely natural in Spain.

A few notes:

• sofá is standard in Spain
• mareado/a is also very common in Spain for dizzy
• the whole structure sounds normal and idiomatic

A Spanish speaker from Spain would understand and naturally use this sentence exactly as written.