Learn Hardcore Swahili: Polisi wanahusika na uchunguzi wa udanganyifu sokoni. - The police are involved in the investigation of fraud at the market.

In practice, nouns referring to people often take human/plural agreement with wa-, even if their noun-class would suggest something else. Polisi refers to people (police officers as a group), so Swahili commonly uses wa-: polisi wanahusika. Zinahusika (class 10 agreement) sounds odd here. If you mean the police institution as a singular body, you can say: Jeshi la Polisi linahusika …

Is polisi singular or plural? How do I refer to one officer?

Polisi is invariant and can mean “the police” (collective) or “police officer(s).” It usually takes plural agreement when referring to officers as a group. For one officer, say askari polisi or afisa wa polisi, or polisi mmoja. For several: polisi wawili, polisi wanne, etc.

What exactly does wanahusika mean, morphologically?

It’s built as wa- + -na- + husika:

wa- = they (3rd person plural subject marker)
-na- = present/ongoing tense
husika = be involved/concerned/be relevant So wanahusika ≈ “they are involved.”

What’s the difference between wanahusika, wamehusika, and walihusika?

wanahusika: present/ongoing state — “are involved.”
wamehusika: present perfect — “have been involved (and still relevant).”
walihusika: simple past — “were involved (then).”

Why husika na …? Could I use katika, or drop the preposition?

Husika typically takes na to mean “be involved with/in”: husika na. Katika can also work, especially in formal style: wanahusika katika uchunguzi. You cannot drop the preposition; husika alone does not take a direct object in this meaning.

In uchunguzi wa udanganyifu, why is it wa and not ya or la?

The connector agrees with the head noun uchunguzi, which is class 14 (u-). Class 14 takes the genitive connector wa. Hence uchunguzi wa udanganyifu (“investigation of fraud”), not uchunguzi ya/la …

What does sokoni mean, and is katika soko or kwenye soko okay?

Sokoni is soko + -ni, the locative suffix, meaning “at/in the market.” Katika soko and kwenye soko are both acceptable; sokoni is the most compact and idiomatic, katika soko a bit more formal, kwenye soko very common in speech.

Does sokoni describe the fraud or the whole activity? How can I make it unambiguous?

At the end of the sentence, sokoni can modify the general location of the involvement or the location of the fraud. To pin it to the fraud, say:

uchunguzi wa udanganyifu uliotokea sokoni (fraud that occurred in the market) To pin it to the police’s location, front it:
Sokoni, polisi wanahusika na uchunguzi wa udanganyifu.

Are there simpler or more common alternatives to udanganyifu?

Common near-synonyms:

utapeli: swindling/scamming (very common in news).
ulaghai: trickery/deception (somewhat formal). Use udanganyifu for “fraud” in general, utapeli for scams, and avoid wizi (theft) unless it’s actually stealing.

Could I say wanashughulikia uchunguzi instead of wanahusika na uchunguzi?

Yes, but the nuance changes:

wanahusika na uchunguzi = they are involved/concerned with the investigation (broad involvement).
wanashughulikia uchunguzi = they are handling/conducting the investigation (they’re actively working on it).

Is it correct to write wana husika as two words?

No. The subject prefix attaches to the verb: wanahusika is one word. Writing wana husika is incorrect spacing.

How do I negate this sentence?

Use the present negative with -i and no -na-:

Polisi hawahusiki na uchunguzi wa udanganyifu sokoni. (They are not involved …) Other useful negatives:
hawakuhusika (they did not get involved — past)
hawajahusika (they have not been involved — present perfect negative)

How do I express a habitual/general truth?

Use the habitual marker hu- (no subject prefix on the verb):

Polisi huhusika na uchunguzi wa udanganyifu sokoni. (Police typically are involved …)

How else could I phrase the same idea?

Jeshi la Polisi linahusika na uchunguzi wa udanganyifu sokoni. (Institution-focused.)
Polisi wanachunguza udanganyifu sokoni. (More direct: “The police are investigating market fraud.”)