Troquei a fronha porque estava um pouco molhada.

Here troquei means I changed or I replaced.

It comes from the verb trocar. In this sentence, trocar a fronha means changing the pillowcase for another one, not just “changing” in a general abstract sense.

A useful comparison:

• trocar a fronha = to change/replace the pillowcase
• mudar can also mean to change, but trocar often feels more like swapping one thing for another.

So Troquei a fronha is very naturally understood as I changed the pillowcase.

Troquei is the 1st person singular of the pretérito perfeito (simple past / preterite).

That means the action is seen as a completed event in the past:

• troco = I change / I am changing
• trocava = I used to change / I was changing
• troquei = I changed

So the speaker is talking about one finished action: they changed the pillowcase.

Because Portuguese often omits subject pronouns when the verb ending already makes the subject clear.

Troquei already tells you the subject is eu (I), so saying Eu troquei a fronha is possible, but not necessary.

Both are correct:

• Troquei a fronha.
• Eu troquei a fronha.

The version without eu is very normal and natural.

Fronha means pillowcase.

Be careful not to confuse it with:

• almofada = pillow
• fronha = pillowcase

So the sentence is about changing the cover of the pillow, not the pillow itself.

A fronha means the pillowcase, while uma fronha means a pillowcase.

Here the speaker is referring to a specific pillowcase, probably one already understood from the situation — for example, the pillowcase on the bed.

So:

• Troquei a fronha = I changed the pillowcase
• Troquei uma fronha = I changed a pillowcase

Using the definite article a is the most natural choice here.

Because porque here means because and introduces the reason.

In Portuguese:

• porque = because
• por que = why / for what reason (in certain structures)
• porquê = the reason / the why
• por quê = a less common separated form used before punctuation

In this sentence, the speaker gives a reason:

• Troquei a fronha porque estava um pouco molhada.
• I changed the pillowcase because it was a little wet.

So porque is the correct form.

Because estava gives a background state: the pillowcase was a bit wet at that time.

Portuguese often uses the imperfect (estava) for descriptions, conditions, or ongoing states in the past.

Here the idea is:

• first, the pillowcase was in the state of being slightly wet
• because of that, the speaker changed it

So:

• estava molhada = it was wet / it was in a wet state
• esteve molhada would sound more like a bounded, completed state and is less natural in this context

The imperfect estava is the normal choice here.

Because molhada agrees with a feminine singular noun.

The noun fronha is feminine singular:

• a fronha
• therefore: molhada

Agreement in Portuguese is very important:

• o lençol molhado = the wet sheet
• a fronha molhada = the wet pillowcase

So molhada matches fronha in gender and number.

Because the adjective molhada is feminine singular, which matches fronha.

In English, you would normally say it was a bit wet. In Portuguese, the subject can be left out if it is understood from context.

So the sentence is understood as:

• Troquei a fronha porque [a fronha] estava um pouco molhada.

A small nuance: in isolation, if the speaker were female, estava molhada could theoretically mean I was wet. But in this sentence, the intended meaning is clearly that the pillowcase was wet, and the adjective agreement supports that reading.

Um pouco means a little or a bit.

It is a fixed expression used to soften or limit the adjective:

• um pouco molhada = a little wet / slightly wet

Even though fronha is feminine, um pouco does not change to uma pouca here. That is because um pouco is functioning as an adverbial expression modifying molhada, not as an adjective agreeing with fronha.

So:

• um pouco cansado/a = a little tired
• um pouco difícil = a little difficult
• um pouco molhada = a little wet

Yes, you could.

Both molhada and húmida relate to wetness, but they are not always identical:

• molhada = wet
• húmida = damp / humid / slightly moist

So:

• molhada suggests clearer wetness
• húmida can sound milder, more like damp

If the pillowcase was noticeably wet, molhada is very natural. If it was only slightly damp, húmida might fit better.