Learn Hardcore Swahili: Unapopiga filimbi uwanjani, watoto wanakimbia. - When you blow the whistle on the field, the children run.

Unapiga is built from u- (you, singular) + -na- (present) + -piga (to hit/strike), so it means you are hitting / you hit.
In this sentence, with filimbi, unapiga filimbi would mean you are blowing the whistle.

Unapopiga adds -po-, a relative marker that often means when / at the time that (or where with places).
So unapopiga filimbi = when you blow the whistle or whenever you blow the whistle.

So:

unapiga filimbi – you are blowing the whistle (simple statement).
unapopiga filimbi – when(ever) you blow the whistle (introduces a time clause).

Why does Swahili say piga filimbi instead of having a single verb meaning to blow a whistle?

Swahili often uses a general verb + noun combination instead of a single specific verb.
Piga literally means to hit/strike, but in many fixed expressions it has wider meanings like make / do / play / blow.

Common patterns:

piga filimbi – blow a whistle
piga simu – make a phone call
piga kelele – make noise
piga picha – take a photo

So piga filimbi is just the normal idiomatic way to say to whistle / to blow a whistle.

What does uwanjani mean, and how is it formed?

The base noun is uwanja – field / pitch / open ground.
Adding -ni to certain nouns makes a locative form meaning in / on / at that place.

So:

uwanja – field
uwanjani – in the field / on the field

Other similar examples:

shule → shuleni – at school
kanisa → kanisani – at church

So uwanjani already includes the idea of in/on, and you don’t need a separate word like in.

Why isn’t there a separate word for when? Why not say something like wakati unapiga filimbi?

The -po- inside unapopiga already expresses when / at the time that.
So unapopiga filimbi itself means when you blow the whistle.

You can say:

Wakati unapiga filimbi uwanjani, watoto wanakimbia.

That is also correct, but then unapiga would usually be used (because wakati already gives the time idea).
Using unapopiga without wakati is more compact and very natural.

Does unapopiga mean present, future, or something like if? How is its time meaning understood?

The -na- part in unapopiga is the present tense/aspect, but with -po- it often has a whenever / when(ever) meaning, which can refer to general time, including future situations.

So Unapopiga filimbi uwanjani, watoto wanakimbia can be understood as:

Whenever you blow the whistle in the field, the children run.

It doesn’t mean if (conditional) as strongly as English if.
For a clearer if idea, Swahili usually uses -ki-, e.g. ukipiga filimbi, watoto watakimbia – if you blow the whistle, the children will run.

What is the difference between unapopiga and ukipiga in a sentence like this?

Both can link two actions, but the nuance is different:

unapopiga filimbi… – when(ever) you blow the whistle…
Neutral, factual timing; we expect that this happens and the second action follows.
ukipiga filimbi… – if/when you blow the whistle…
More conditional or hypothetical: it suggests if this happens (maybe it will, maybe it won’t), then…

So:

Unapopiga filimbi uwanjani, watoto wanakimbia.
Focus on a regular pattern: whenever that happens, they run.
Ukimpiga filimbi uwanjani, watoto watakimbia.
Sounds more like if you blow the whistle, they will (then) run.

Why is there a comma between the two parts instead of a word like then or so?

In Swahili, two related clauses are often just put next to each other, sometimes with a comma in writing, with no extra linking word.

Unapopiga filimbi uwanjani, watoto wanakimbia.
literally: When you blow the whistle in the field, children run.

You can add a linking word if you want to emphasize the result or sequence:

Unapopiga filimbi uwanjani, basi watoto wanakimbia. (then the children run)
… hivyo watoto wanakimbia. (so the children run)

But it is not required; the relationship is clear from the structure and context.

Why is it wanakimbia and not something like watakimbia (will run)?

-na- in wanakimbia marks present, but it also covers:

actions happening now, and
regular or predictable reactions (habitual/general truth).

So watoto wanakimbia here means the children run as a regular, expected response whenever the condition is met.
English sometimes prefers will in this kind of sentence, but Swahili is comfortable using the present.

If you really want to stress the future result (a specific future occasion), you can say:

Unapopiga filimbi uwanjani, watoto watakimbia. – when you blow the whistle (that time), the children will run.

Why doesn’t Swahili use words like the for children and field?

Swahili does not have articles like the or a.
Nouns like watoto and uwanja can mean children / the children or a field / the field depending on context.

So:

watoto can be children or the children
uwanjani can be in a field or in the field

Definiteness (whether it’s the or just some) is understood from the situation or from earlier sentences, not from a separate word.

Is wewe (you) missing here? Why isn’t it written?

Swahili usually does not need a separate subject pronoun, because the subject is already marked on the verb.

In unapopiga:

u- = you (singular)
-na- = present
-po- = when
-piga = hit/blow

So wewe unapopiga filimbi… would be more emphatic, like you, when you blow the whistle…
In neutral sentences, the subject pronoun (wewe, mimi, sisi, etc.) is normally omitted unless you want to stress it.

Can this sentence be reordered, for example starting with watoto?

Yes. Swahili word order is fairly flexible with larger phrases. You can say:

Unapopiga filimbi uwanjani, watoto wanakimbia.
Watoto wanakimbia unapopiga filimbi uwanjani.

Both are correct.
Putting the unapopiga… part first puts more emphasis on the condition or time (when you blow the whistle…).
Starting with watoto wanakimbia emphasizes the children’s reaction, then explains when it happens.

How is wanakimbia built, and what aspect of running does it express here?

Wanakimbia is formed as:

wa- – they (class 2: watoto)
-na- – present tense/aspect
-kimbia – to run

So wanakimbia means they are running / they run.
In this sentence, it expresses a regular reaction: every time the condition is met, the children run (not just once, and not only right this second).