Unapozungumza na wazazi wako, uwe mstaarabu.

It can be broken down like this:

• u- = you (singular)
• -na- = present/habitual marker
• -po- = when / at the time when
• zungumza = speak / talk

So unapozungumza means something like when you speak or whenever you are speaking.

In this sentence, -po- gives a time meaning: when.

So:

• unazungumza = you are speaking
• unapozungumza = when you are speaking / when you speak

This is a very common Swahili pattern for time clauses.

Yes. Ukizungumza na wazazi wako, uwe mstaarabu is also natural.

The difference is roughly:

• unapo- = more clearly when / whenever
• uki- = often if / when

In many everyday situations, both work. Here, unapozungumza sounds a little more explicitly like a general rule about the time when you speak to your parents.

Yes, na often means and or with, but after verbs like kuzungumza it is also used for the person you are speaking with/to.

So:

• kuzungumza na mtu = to speak with someone / to talk to someone

Swahili does not need a separate word exactly matching English to in this structure.

Wazazi means parents.
Its singular is mzazi, meaning parent.

This follows a very common noun-class pattern for people:

• m- in the singular
• wa- in the plural

So:

• mzazi = parent
• wazazi = parents

In Swahili, possessives agree with the noun class of the noun they describe.

The possessive stem for your is -ako.
With wazazi, it takes the appropriate agreement and becomes wako.

So:

• mzazi wako = your parent
• wazazi wako = your parents

A learner may expect one single word for your, but in Swahili the form changes depending on the noun class.

Uwe is the subjunctive form of kuwa (to be).

Swahili often uses the subjunctive for:

• commands
• advice
• encouragement
• wishes

So uwe mstaarabu means be polite / be well-mannered.

Using kuwa here would not work the same way, because kuwa is the infinitive, meaning to be, not a command or instruction.

In this context, mstaarabu means something like:

• polite
• well-mannered
• respectful
• decent in behavior

Its broader sense can also be civilized or refined, but in a sentence about talking to your parents, the natural meaning is well-mannered/respectful.

Because the sentence is addressing one person.

The person being told to behave properly is you singular, shown by:

• u- in unapozungumza
• uwe

So the description also stays singular: mstaarabu.

If you were addressing several people, you would normally change the sentence, for example:

• Mnapozungumza na wazazi wenu, muwe wastaarabu.

It is speaking to one person.

Clues:

• u- = singular you
• wako = your (singular)
• uwe = singular subjunctive be

If the speaker were talking to several people, the forms would change to plural, such as mna-, wenu, and muwe.

Both relate to speaking, but they are not exactly the same.

• sema often means say or speak
• zungumza often means talk / converse / speak with

Because the sentence is about how you speak when interacting with your parents, zungumza na is very natural.

So:

• sema focuses more on saying something
• zungumza focuses more on talking or conversing

Yes. You could also say:

• Uwe mstaarabu unapozungumza na wazazi wako.

That is still grammatical.

The original order puts the when clause first, which is a natural way to set the situation before giving the advice.