Se a fralda estiver suja, a Ana muda-a antes de a deitar no berço.

Estiver is the future subjunctive of estar.

After se meaning if, Portuguese often uses the future subjunctive when the condition is about something possible or not yet confirmed:

• Se a fralda estiver suja... = If the diaper is dirty / if the diaper turns out to be dirty

Using está would sound more like you are talking about a condition seen as present and factual right now. In this sentence, the idea is more general: whenever / if it happens to be dirty.

It means If the diaper is dirty.

Literally, English does not really show the same subjunctive distinction here, so the natural translation is just if the diaper is dirty.

The Portuguese form adds the idea of a future or uncertain condition:

• if, when checked, the diaper is dirty
• if it turns out to be dirty

In European Portuguese, it is very common to use the definite article before a person’s name:

• a Ana
• o João

This does not usually get translated into English. So a Ana simply means Ana.

This is especially normal in everyday spoken Portuguese in Portugal.

Because it agrees with fralda, which is a feminine singular noun.

So:

• a fralda → feminine singular
• suja → feminine singular adjective

If the noun were masculine, the adjective would change:

• o chão sujo = the dirty floor

Because -a is a clitic object pronoun attached to the verb.

In European Portuguese, when nothing forces the pronoun to come before the verb, it often goes after the verb. This is called enclisis:

• muda-a = changes it

The hyphen is required in standard spelling when the pronoun is attached like this.

It refers to a fralda.

So:

• a fralda = the diaper
• muda-a = changes it

The pronoun is a because fralda is a feminine singular noun.

That a is also an object pronoun, but it refers to someone else, not to a fralda.

Here it means something like her or it, depending on the noun understood from the wider context. In a sentence like this, it normally refers to the baby/child being put into the crib.

So the structure is:

• antes de a deitar no berço = before putting her in the crib

Portuguese often leaves the noun itself unstated if it is already clear from context.

Not necessarily.

The pronoun agrees with the grammatical gender of the noun it refers to. So it could refer to something like:

• a bebé
• a criança
• a menina

Even if English would say the baby, Portuguese still has to choose a grammatical gender.

If the noun were masculine, you would get:

• antes de o deitar no berço

Because with a preposition + infinitive structure, European Portuguese very often places the clitic pronoun before the infinitive:

• antes de a deitar
• para o ajudar
• sem me avisar

This is very natural in Portugal.

You may also see forms like antes de deitá-la, but antes de a deitar is a very normal European Portuguese pattern.

Here deitar means to lay down or to put to bed.

So:

• deitar no berço = to lay/put in the crib

It is not reflexive here, because Ana is putting someone else down.

Compare:

• deitar a criança = to put the child down
• deitar-se = to lie down / go to bed oneself

No berço means in the crib.

No is a contraction of:

• em
  • o = no

So:

• em o berço → no berço

This kind of contraction is very common in Portuguese:

• na = em + a
• do = de + o
• da = de + a

Yes. In this context, mudar a fralda means to change the diaper.

That is a normal and idiomatic way to say it in Portuguese. So:

• a Ana muda-a = Ana changes it

The verb is being used transitively with fralda as the direct object.

You can get close, but a fully natural English version is better.

A near-literal breakdown is:

• Se = if
• a fralda = the diaper
• estiver suja = is dirty / turns out to be dirty
• a Ana = Ana
• muda-a = changes it
• antes de a deitar no berço = before putting her in the crib

A natural English translation would be:

If the diaper is dirty, Ana changes it before putting her in the crib.

The main places where Portuguese works differently are:

• the future subjunctive in estiver
• the use of articles with names
• the use of clitic pronouns like -a and a deitar