Asipovaa kaptula safi baada ya kuoga, mtoto atalalamika tena.

Asipovaa breaks down as:

• a- = he/she
• -si- = negative
• -po- = if/when
• vaa = wear / put on

So asipovaa means if he/she does not wear.

This is a common Swahili negative conditional pattern:

• nisipofanya = if I do not do
• usipoenda = if you do not go
• wasipokuja = if they do not come

Because Swahili normally puts the subject inside the verb.

For example:

• a-si-po-vaa = he/she does not wear
• a-ta-lalamika = he/she will complain

That a- already means he/she.
So Swahili often does not need a separate pronoun like English does.

The difference is the tense marker:

• a-ta-lalamika = he/she will complain
• a-na-lalamika = he/she is complaining / complains

So in this sentence, atalalamika is future: will complain.

Yes, in a way it is, but that is normal in Swahili.

Swahili verbs usually still carry a subject marker even when the noun subject is stated explicitly.

So:

• mtoto atalalamika
  literally: the child he/she-will-complain

This is standard grammar, not bad repetition. The noun mtoto names the subject, and the verb agrees with it.

• baada ya = after
• kuoga = to bathe / to wash oneself

So baada ya kuoga literally means after bathing or after to bathe, but in natural English we say after bathing or after taking a bath.

After baada ya, Swahili commonly uses the infinitive form with ku-:

• baada ya kula = after eating
• baada ya kusoma = after reading/studying
• baada ya kuoga = after bathing

Often it can mean either, depending on context.

With clothes, vaa can cover both the idea of:

• put on
• wear

So asipovaa kaptula safi can be understood as:

• if he/she does not put on clean shorts or
• if he/she does not wear clean shorts

English separates those ideas more clearly than Swahili often does.

Because Swahili treats kaptula as a normal noun referring to a garment, even though English uses a plural form shorts.

So kaptula can mean:

• shorts
• a pair of shorts

This is not unusual when moving between languages.
English has some clothing words that are grammatically plural, but Swahili does not have to match that pattern.

Safi is one of the adjectives that usually stays the same form across noun classes.

So you can get:

• mtoto safi = a clean child
• nguo safi = clean clothes
• kaptula safi = clean shorts

Many Swahili adjectives do agree with noun class, but some common adjectives, especially loanword-type adjectives like safi, are often invariable.

You know from context.

Grammatically, a- just means he/she. It does not by itself tell you exactly which person. In context, it may refer to:

• the child
• some other previously mentioned person

If this sentence appears by itself, many learners will naturally assume it refers to the child, but strictly speaking, context decides.

Tena means again or once more.

So:

• atalalamika tena = will complain again

It is often placed after the verb or near the end of the clause, just as it is here.

Because Swahili does not have articles like English a/an/the.

So:

• mtoto can mean a child or the child
• kaptula safi can mean clean shorts or the clean shorts, depending on context

English forces you to choose an article, but Swahili usually leaves that to context.

Yes. Swahili can also put the main clause first, for example:

• Mtoto atalalamika tena asipovaa kaptula safi baada ya kuoga.

That still means the same thing.
Starting with the if-clause is very natural, but reversing the order is also possible.