No guardo la toalla hasta que se seque.

Here guardar means to put away, to store, or to put back in its place.

So No guardo la toalla... means I don’t put the towel away..., not I don’t keep the towel in a general sense.

In Spanish, no normally goes directly before the conjugated verb to make the sentence negative.

• Guardo la toalla = I put the towel away.
• No guardo la toalla = I don’t put the towel away.

That word order is the standard one.

Spanish often uses the present tense for:

• habits
• routines
• general rules

So No guardo la toalla hasta que se seque sounds like a general habit: I don’t put the towel away until it dries.

If you wanted to talk about one specific future situation, No guardaré la toalla hasta que se seque would also be possible.

Spanish very often uses the definite article (el, la, los, las) where English might use a possessive like my or your, especially when it is already obvious whose thing it is.

So la toalla is perfectly natural here.

Also, a singular countable noun usually needs some kind of determiner in Spanish, so toalla by itself would not sound right here.

Hasta que means until.

It introduces the point at which the first action changes:

• No guardo la toalla = I don’t put the towel away
• hasta que se seque = until it dries

So the whole sentence means that the towel is not put away before that moment.

Because after hasta que, Spanish uses the subjunctive when the action is still pending or future from the speaker’s point of view.

Here, the towel is not dry yet, so Spanish says:

• hasta que se seque

not

• hasta que se seca

A useful rule is:

• hasta que + subjunctive for something that has not happened yet
• hasta que + indicative for something seen as an actual or completed fact in a different context

It is not a typo.

There are two different parts here:

• se = the reflexive/pronominal pronoun
• seque = the verb form

The verb is secarse in this use, meaning to dry or to get dry.

So se seque means it dries / it gets dry, in the subjunctive.

The subject is la toalla:

• la toalla se seque = the towel dries

In natural English, we just say until it dries.

Because secar and secarse are used differently:

• secar = to dry something
  • Seco la toalla. = I dry the towel.
• secarse = to dry / to get dry
  • La toalla se seca. = The towel dries / gets dry.

In your sentence, the towel is the thing becoming dry, so secarse is the natural choice.

This is a spelling change to keep the same sound.

The verb is secar. In forms before e, Spanish changes c to qu so that the sound stays hard, like k.

• secar
• seque

Without the u, ce would sound different.

This happens with many -car verbs:

• buscar → busque
• tocar → toque
• secar → seque

Yes. That is also natural Spanish.

There is a small difference in focus:

• hasta que se seque = until it dries
• hasta que esté seca = until it is dry

The first focuses a bit more on the process/result of drying. The second focuses more on the state of being dry.

Both are correct and common.

There are two different subjects:

• guardo → the subject is an implied yo = I
• se seque → the subject is la toalla = the towel

So the structure is:

• I don’t put away
• the towel
• until the towel dries

That shift in subject is very common in Spanish sentences with hasta que.