Minha filha tirou a casca do abacaxi, mas deixou a casca do mamão na mesa.

Because possessive adjectives agree with the noun that follows.

• filha = feminine singular
• so the possessive must also be feminine singular: minha

Compare:

• meu filho = my son
• minha filha = my daughter

It does not agree with the speaker’s gender. A man and a woman would both say minha filha.

In Brazilian Portuguese, it is very common to use a possessive directly with a family noun:

• minha filha
• meu pai
• nossa mãe

You can also hear a minha filha in some varieties or for emphasis, but minha filha sounds very natural in Brazil.

So both can exist, but the version without the article is extremely common.

Tirou is the preterite form of tirar, meaning something like:

• removed
• took off
• took away

Here, tirou a casca do abacaxi means she removed the pineapple peel.

The preterite is used because this is a completed action in the past.

• tirar = to remove / take off
• tirou = removed / took off

Because casca is a feminine noun in Portuguese.

So you use:

• a casca = the peel / the skin

That is also why later you get:

• a casca do abacaxi
• a casca do mamão

The article a matches the gender of casca, not the fruit.

Because do is a contraction of:

• de + o = do

Here, de expresses the idea of of:

• a casca do abacaxi = the peel of the pineapple
• a casca do mamão = the peel of the papaya

This is very common in Portuguese.

Other similar contractions:

• de + a = da
• de + os = dos
• de + as = das

Yes, abacaxi is masculine: o abacaxi.

In Portuguese, noun gender is not always predictable from the ending. A word ending in -i can still be masculine.

So:

• o abacaxi
• do abacaxi

Likewise:

• o mamão
• do mamão

You usually need to learn the gender together with the noun.

Because deixou do mamão would be incomplete and unnatural here.

Portuguese normally repeats the noun for clarity:

• tirou a casca do abacaxi, mas deixou a casca do mamão na mesa

You could shorten it in some contexts:

• ...mas deixou a do mamão na mesa

That works because a stands in for a casca. But the full version is clearer and more neutral.

Here, deixou means left in the sense of left something somewhere.

• deixou a casca do mamão na mesa = left the papaya peel on the table

It is the preterite of deixar, so again it describes a completed past action.

Be careful: deixar can have several meanings depending on context, such as:

• to leave
• to let
• to allow

In this sentence, it clearly means left.

Because na is a contraction of:

• em + a = na

Since mesa is feminine:

• a mesa = the table
• na mesa = on the table / in the table contextually on the table

This is another very common Portuguese contraction.

Related forms:

• no = em + o
• nos = em + os
• nas = em + as

It goes with deixou.

The idea is:

• she removed the pineapple peel,
• but she left the papaya peel on the table.

So na mesa tells you where she left it.

Structurally, it modifies the action of deixou, not the noun mamão.

Yes. A very natural alternative would be:

• descascou o abacaxi

That literally means peeled the pineapple.

So these are close in meaning:

• tirou a casca do abacaxi
• descascou o abacaxi

The sentence you were given is slightly more explicit because it directly mentions a casca.

Because mas means but, and it connects two contrasting clauses:

• Minha filha tirou a casca do abacaxi
• mas deixou a casca do mamão na mesa

Using a comma before mas is standard and natural here, just as in English you often write a comma before but when joining two full clauses.

The ending -ão is a very common nasal ending in Portuguese.

So mamão is not pronounced like an English ow sound. The final part is nasal, roughly like ma-MOWN with the last syllable nasalized.

A few helpful notes:

• stress is on the last syllable: ma-MÃO
• the -ão sound appears in many words:
  • pão
  • mão
  • não

This ending often takes practice for English speakers because English does not use nasal vowels in the same way.

Yes. Both tirou and deixou are in the preterite.

• tirou = removed
• deixou = left

The preterite is used because the sentence tells a sequence of completed actions in the past.

If the sentence were describing habitual or ongoing background action, Portuguese might use a different past tense, but here the preterite is the natural choice.