Deixa o casaco no cabide até que a fronha esteja seca.

Yes. Deixa is an affirmative imperative form, used to give a command or instruction.

In European Portuguese:

• deixa = command to tu (informal singular)
• deixe = command to você / o senhor / a senhora (more formal singular)

So this sentence is speaking to one person informally.

It is understood, not stated.

Portuguese often omits subject pronouns when the verb form already makes the person clear. Here, deixa already tells you the speaker is addressing tu.

So the sentence does not need an explicit tu.

Here it mainly means leave or leave in place, though in natural English it may sometimes feel close to put depending on context.

For example:

• Deixa o livro na mesa. = Leave the book on the table.
• Deixa o casaco no cabide. = Leave the coat on the hanger.

So the idea is not just moving it there, but keeping it there.

Portuguese often uses the definite article where English would use a possessive like your.

When the owner is obvious from the situation, Portuguese commonly says:

• Veste o casaco. = Put on your coat.
• Lava as mãos. = Wash your hands.

So o casaco can naturally mean your coat if the context makes that clear.

No is a contraction:

• em + o = no

So:

• no cabide = in/on the hanger, depending on what sounds natural in English

In this sentence, the best natural translation is usually on the hanger.

Cabide usually means a clothes hanger.

Depending on context, it can sometimes refer more generally to something used for hanging clothes, but the most common meaning is the ordinary hanger you put clothes on.

So o casaco no cabide suggests the coat is hanging on a hanger.

Fronha means pillowcase.

It is a normal, everyday word in Portugal. It refers specifically to the fabric cover for a pillow, not to:

• lençol = sheet
• cobertor = blanket
• edredão = duvet

So a fronha esteja seca means the pillowcase is dry.

Because até que introduces a full clause with its own verb:

• até que a fronha esteja seca

Here you have:

• a subject: a fronha
• a verb: esteja

By contrast, plain até is used before a noun, adverb, or infinitive, for example:

• até amanhã
• até secar
• até a fronha secar

So:

• até que
  • finite verb clause
• até
  • noun / adverb / infinitive

Because esteja is the present subjunctive of estar, and after até que Portuguese normally uses the subjunctive when referring to something future, expected, or not yet achieved.

In this sentence, the pillowcase is not dry yet at the moment of speaking. The sentence is talking about the point in the future when that condition becomes true.

So:

• até que a fronha esteja seca = until the pillowcase is dry

Using está would sound wrong here because that would suggest a straightforward present fact rather than a future condition.

Because dry here is a temporary state or a resulting condition, and Portuguese normally uses estar for that.

• estar seca = to be dry, in the sense of having dried
• ser seca would suggest a more inherent characteristic, or even a different meaning depending on context

So for something like laundry, clothes, or a pillowcase becoming dry, estar is the natural choice.

Because seca is an adjective agreeing with a fronha, which is feminine singular.

Agreement in Portuguese:

• a fronha seca
• o casaco seco
• as fronhas secas
• os casacos secos

So the ending changes to match the noun.

Yes, absolutely.

A very natural alternative is:

• Deixa o casaco no cabide até a fronha secar.

That structure uses the infinitive secar instead of até que + subjunctive.

The difference is small:

• até que a fronha esteja seca focuses on the state of being dry
• até a fronha secar focuses a bit more on the process/event of drying

Both are natural and correct.