Learn Hardcore Swahili: Polisi wanachunguza kesi sokoni. - The police are investigating a case at the market.

The police as an institution or group (plural idea, like English “the police”)
A police officer, especially in informal speech, though more precise words are:
- askari polisi – police officer
- afisa polisi – police officer (often more formal/senior)

In your sentence, Polisi wanachunguza..., the verb uses the plural subject prefix wa-, so polisi here is clearly being used as a plural: the police (they) are investigating…

How is the verb wanachunguza built up morphologically?

Wanachunguza can be broken down like this:

wa- – subject prefix for they (3rd person plural)
-na- – present tense marker (often “are doing” / “do” right now)
-chunguza – verb root meaning investigate, examine, look into

So:

wanachunguza = wa- (they) + -na- (present) + -chunguza (investigate)
→ they are investigating

Does -na- in wanachunguza mean “are investigating” (progressive) or just “investigate” (simple present)?

The Swahili -na- tense is flexible. It usually covers what English expresses with both:

Present continuous: they are investigating
Simple present (current, regular activity): they investigate

In this sentence, English speakers would most naturally translate it as “are investigating”, but context could also allow a habitual reading, like The police investigate the case at the market (as a routine).

So:

wanachunguza ≈ “they (are) investigate(ing)”
Actual translation depends on context, not on a different tense form.

Why is there no word for “the” or “a” before kesi?

Swahili does not have separate words for “a / an” or “the”.
The noun alone covers all of these possibilities:

kesi can mean a case, the case, cases, or the cases, depending on context.

If you want to be more specific, you use other words:

kesi hiyo – that case / the case (just mentioned)
kesi moja – one case / a (single) case
kesi zote – all the cases

In this sentence, kesi is best understood as “a case” or “the case” from context alone.

What does kesi mean exactly here?

Kesi is a borrowed word (from English “case”) and commonly means:

a legal case, court case
more generally, a case or matter that is being investigated

In Polisi wanachunguza kesi sokoni, it most naturally means:

a case / matter that the police are investigating, possibly (but not necessarily) a legal or criminal case.

What is the difference between soko and sokoni?

Both come from the same noun, but -ni adds a locative meaning.

soko – market (the basic noun)
sokoni – at the market / in the market / to the market (locative form)

The suffix -ni often indicates location:

nyumba – house → nyumbani – at home
shule – school → shuleni – at school

So in this sentence, sokoni tells you where the investigation is taking place: at the market / in the market.

Could I say katika soko instead of sokoni? What’s the difference?

Yes, you could say:

Polisi wanachunguza kesi katika soko.

This is grammatically correct and means “The police are investigating a case in the market.”

The difference:

sokoni – more natural and common in everyday speech; very idiomatic.
katika soko – also correct, sometimes sounds a bit more formal or neutral, and uses the preposition katika (“in/inside/within”).

In most spoken situations, sokoni is preferred and sounds more native.

Why is the place word sokoni at the end of the sentence? Can it go somewhere else?

Swahili word order is fairly flexible, but a very common pattern is:

Subject – Verb – Object – Place/Time

So your sentence:

Polisi (subject)
wanachunguza (verb)
kesi (object)
sokoni (place)

You can move sokoni for emphasis or style:

Polisi sokoni wanachunguza kesi. – Emphasis on the police at the market (contrast with police elsewhere).
Sokoni, polisi wanachunguza kesi. – “At the market, the police are investigating a case.” (fronted for emphasis or scene-setting)

But Polisi wanachunguza kesi sokoni is the most neutral and typical order.

Why doesn’t the verb have an object marker, like wanaichunguza kesi?

Swahili has object markers (OMs) that can be inserted into the verb.
For kesi (class 9/10), the object marker is i-.

So in theory, you could have:

wana-i-chunguza – “they are investigating it”

But you normally use an object marker when:

the object is already known, specific, or has been mentioned before, and
you often don’t repeat the full noun, or you want to emphasize it.

Examples:

Polisi wanaichunguza. – The police are investigating it (the case we know about).
Polisi wanaichunguza kesi hiyo. – The police are investigating that case (emphasis on that case).

In your sentence, the bare noun kesi is perfectly natural and may feel more neutral, especially if the case hasn’t been clearly identified in the conversation yet.

Does sokoni mean “at the market” or “in the market” or “to the market”?

Sokoni can cover all of these, depending on context:

at the market
in the market
to the market (especially with movement verbs like ninaenda sokoni – “I am going to the market”)

Here, with wanachunguza (are investigating), the most natural reading is:

at the market or in the market

English has to choose a specific preposition; Swahili’s -ni locative is more general.

If I want to say “The police are investigating that case at the market,” how would I change this sentence?

You can add a demonstrative to kesi to specify “that case”:

Polisi wanachunguza kesi hiyo sokoni.
– The police are investigating that case at the market.

Breakdown:

kesi – case
hiyo – that (near the listener / just mentioned)
kesi hiyo – that case
sokoni – at the market

So you are now clearly talking about a specific, known case.