Learn Hardcore Swahili: Mfuko wangu uliibiwa sokoni jana. - My bag was stolen at the market yesterday.

Mfuko – bag / pocket / sack; the subject noun (what the sentence is about). Noun class 3 (m-/mi-).
wangu – my; possessive adjective agreeing with mfuko’s noun class.
uliibiwa – was stolen; verb in the passive, agreeing with mfuko.
sokoni – at the market; soko (market) + -ni locative ending = “at/in the market”.
jana – yesterday; time adverb.

So the structure is basically: [My bag] [was stolen] [at the market] [yesterday].

How is the verb uliibiwa built up, piece by piece?

Uliibiwa can be divided like this:

u- – subject prefix for noun class 3 singular (agreeing with mfuko).
-li- – past tense marker (simple past).
-ibiwa – passive stem from the verb iba (to steal).

So uliibiwa literally encodes:
u- (it) + -li- (past) + -ibiwa (be stolen) → “it was stolen”.

Compare:

Mfuko wangu unaibiwa. – My bag is being stolen / is in the process of being stolen.
Mfuko wangu utaibiwa. – My bag will be stolen.

Why does the verb start with u-? Why not a- or something else?

The u- at the beginning of uliibiwa is the subject agreement for the noun class of mfuko.

Mfuko belongs to noun class 3 (m-/mi-).
Class 3 uses u- as its subject prefix in the 3rd person singular.

So:

Mfuko wangu uliibiwa. – My bag was stolen. (class 3 → u-)
Mifuko yangu iliibiwa. – My bags were stolen. (class 4 → i-)

If the subject were a class 1 human noun, you’d see a- instead:

Mtu aliiba mfuko wangu. – A person stole my bag.
(class 1 mtu → a-li-iba)

Why is it wangu and not yangu after mfuko?

Possessive forms in Swahili have to agree with the noun class of the noun they describe.

Mfuko is class 3 (m-/mi-), and class 3 singular takes w- for the “my” form: wangu.
Its plural mifuko is class 4, which takes y- for “my”: yangu.

So:

mfuko wangu – my bag (class 3 → wangu)
mifuko yangu – my bags (class 4 → yangu)

You’ll see the same pattern with other class 3/4 nouns:

mti wangu / miti yangu – my tree / my trees
mwili wangu / miili yangu – my body / my bodies

Why is the passive (uliibiwa, “was stolen”) used instead of an active form?

Swahili often uses the passive when:

the doer (thief) is unknown, not important, or obvious, or
the focus is on the thing affected, not on who did it.

Here the important information is that your bag was stolen, not who stole it, so:

Mfuko wangu uliibiwa sokoni jana. – My bag was stolen at the market yesterday.

An active version would be:

Mtu aliiba mfuko wangu sokoni jana. – Somebody stole my bag at the market yesterday.
Mwizi aliiba mfuko wangu sokoni jana. – A thief stole my bag at the market yesterday.

You can also add the thief to the passive sentence:

Mfuko wangu uliibiwa na mwizi sokoni jana.
My bag was stolen by a thief at the market yesterday.

What exactly is sokoni, and how is it different from soko or kwenye soko?

Soko = market (a noun).
Adding -ni makes a locative form: sokoni = at the market / in the market.

So:

Niko sokoni. – I am at the market.
Mfuko wangu uliibiwa sokoni jana. – My bag was stolen at the market yesterday.

You can often replace sokoni with:

kwenye soko or katika soko – both mean in/at the market too.

In everyday speech, the -ni form (sokoni, nyumbani, shuleni, kazini) is extremely common and usually the most natural choice.

Can I move jana or sokoni to a different place in the sentence? Does the meaning change?

Yes, you can move them around; the basic meaning stays the same, but the emphasis can shift slightly.

All of these are grammatical:

Mfuko wangu uliibiwa sokoni jana.
Mfuko wangu uliibiwa jana sokoni.
Jana mfuko wangu uliibiwa sokoni.

Tendencies:

Time words like jana are very often at the beginning or end of the sentence.
Putting jana at the very beginning (Jana mfuko wangu…) slightly emphasizes the time: “Yesterday, (this happened)…”
Putting sokoni earlier or later mostly just changes rhythm, not meaning.

The original sentence, with sokoni jana at the end, is very natural.

How would I say “My bag has been stolen” (without saying when)? Is that different from uliibiwa?

Yes, that uses a different tense:

Mfuko wangu umeibiwa. – My bag has been stolen.

Here:

u- – class 3 subject marker (for mfuko)
-me- – perfect aspect (roughly like the English present perfect)
-ibiwa – passive stem of iba (to steal)

Difference in usage:

uliibiwa (past -li-) – a completed event at a specific past time, especially with words like jana, juzi, mwaka jana.
- Mfuko wangu uliibiwa sokoni jana. – It was stolen yesterday.
umeibiwa (perfect -me-) – a recent or relevant past event whose result still matters now; often used without a specific time adverb.
- Mfuko wangu umeibiwa! – My bag has been stolen! (I’ve just discovered it.)

It’s unusual to say umeibiwa jana; normally you’d use uliibiwa jana with jana.

How do I say “My bags were stolen at the market yesterday”? What changes in the Swahili sentence?

You mainly need to:

Make mfuko plural → mifuko (class 4).
Change the agreement on the possessive and the verb.

The sentence becomes:

Mifuko yangu iliibiwa sokoni jana.
My bags were stolen at the market yesterday.

Breakdown:

Mifuko – bags (class 4).
yangu – my (class-4 possessive form).
iliibiwa – verb agreeing with class 4:
- i- – subject prefix for class 4
- -li- – past tense
- -ibiwa – passive stem of iba

Compare:

Mfuko wangu uliibiwa… – (one) bag was stolen.
Mifuko yangu iliibiwa… – (several) bags were stolen.

Does mfuko mean “bag” or “pocket”? Are there other common words for “bag”?

Mfuko can mean:

bag, sack (e.g. mfuko wa plastiki – plastic bag),
pocket (e.g. mfuko wa suruali – trouser pocket),
metaphorical “fund” or “pool of money” (mfuko wa pensheni – pension fund).

Other common “bag” words:

begi – a (modern) bag, often a backpack or suitcase (from English bag).
mkoba – handbag, purse, shoulder bag.
gunia – a big sack (often for grain, potatoes, etc.).

In context, Mfuko wangu uliibiwa sokoni jana is usually understood as my bag was stolen at the market yesterday, unless you’ve clearly been talking about clothes pockets.

How should I pronounce uliibiwa, especially the two i letters?

Orthographically uliibiwa has lii, but in speech:

You don’t pause between the two i’s as separate syllables.
It normally comes out as a single long [i] sound.

A careful syllable breakdown is roughly:

u-lii-bi-wa

So you might hear it as something like “oo-LEE-bi-wa”, with LEE slightly lengthened. You do not insert a consonant between the i’s (no uli-yibiwa or uliwibiwa).

How would I add “by a thief” to the sentence in Swahili?

You use na to introduce the agent (the doer) in a passive sentence:

Mfuko wangu uliibiwa na mwizi sokoni jana.
My bag was stolen by a thief at the market yesterday.

Structure:

uliibiwa – was stolen (passive)
na mwizi – by a thief (agent phrase)

In many real situations, though, speakers just say:

Mfuko wangu uliibiwa sokoni jana.

because the thief is either unknown or obvious from context.