Depois da aula, eu lavei o uniforme e preparei outro lanche para amanhã.

Because da is the contraction of de + a.

• depois de = after
• a aula = the class / the lesson
• depois da aula = after the class

Portuguese very often contracts a preposition plus an article:

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

So depois da aula is the normal form.

It can mean either, depending on context.

Literally, it is after the class/lesson, but in natural English you will often translate it as:

• after class
• after the lesson
• after school only if the broader context makes that sense

In this sentence, after class is probably the most natural translation.

Yes, it could.

Portuguese often drops subject pronouns because the verb ending already shows who the subject is. So both of these are correct:

• Depois da aula, eu lavei o uniforme...
• Depois da aula, lavei o uniforme...

Including eu can do a few things:

• make the subject extra clear
• add a little emphasis
• sound slightly more explicit or natural in some contexts

So eu is optional here, not required.

They are both in the preterite, the tense usually used for completed actions in the past.

• lavar = to wash
  eu lavei = I washed
• preparar = to prepare
  eu preparei = I prepared

This tense is used because the speaker is describing finished actions:

• they washed the uniform
• they prepared another snack

For many -ar verbs, the eu form in the preterite ends in -ei:

• falar → falei
• lavar → lavei
• preparar → preparei

Uniforme is masculine in Portuguese, so it takes o:

• o uniforme

This is a good reminder that word endings do not always tell you the gender. Even though uniforme ends in -e, it is masculine.

You just learn it together with the article:

• o uniforme
• not a uniforme

Portuguese uses definite articles more often than English.

In English, you can say I washed my uniform or sometimes just I washed the uniform, depending on context. In Portuguese, when talking about a specific known thing, using the article is very normal:

• lavei o uniforme

It sounds like a specific uniform already known in the situation, such as a school, work, or sports uniform.

It can suggest either meaning, depending on context.

Outro can mean:

• another
• one more
• a different

So preparei outro lanche could mean:

• I prepared another snack
• I prepared a different snack

In many everyday situations, English another snack is the best translation.

Usually no, at least in Brazilian Portuguese.

In Brazil:

• lanche usually means snack, light meal, or sometimes even a sandwich
• almoço means lunch

So a learner should be careful not to assume that lanche = lunch just because they look similar.

Para amanhã means for tomorrow.

It shows the purpose or intended time of the snack:

• preparei outro lanche para amanhã = I prepared another snack for tomorrow

This does not mean the preparation happened tomorrow. The preparation happened in the past, and the snack is intended for tomorrow.

That is why para is important here: it connects the snack to tomorrow.

Yes. In spoken Brazilian Portuguese, pra is extremely common and natural.

So you will often hear:

• preparei outro lanche pra amanhã

This means exactly the same thing as para amanhã. The difference is mainly style:

• para = more neutral, careful, or written
• pra = very common in speech, more informal

Because Depois da aula is an introductory time expression.

The comma helps separate that opening phrase from the main part of the sentence:

• Depois da aula, eu lavei o uniforme...

In Portuguese, this comma is very common and natural. It works much like English:

• After class, I washed the uniform...

Sometimes commas with short introductory phrases can vary a little in informal writing, but here the comma is perfectly standard.

Yes, Portuguese allows some flexibility in word order.

For example:

• Eu lavei o uniforme e preparei outro lanche para amanhã depois da aula.
• Depois da aula, preparei outro lanche para amanhã e lavei o uniforme.

The basic meaning stays similar, but the emphasis changes a little.

The original sentence is very natural because it starts with the time phrase and then lists the two completed actions in a clear order.