Mfungwa yuko gerezani.

Because this sentence is talking about location.

• ni is used to identify or equate something:
  Yeye ni mfungwa = He/She is a prisoner
• yuko is used to say that someone is in a place / is present:
  Mfungwa yuko gerezani = The prisoner is in prison

So in this sentence, yuko is the correct kind of is.

Gerezani is built from gereza + -ni.

• gereza = prison
• -ni = a locative suffix, often meaning in, at, to, on, depending on context

So gerezani means in prison / at the prison.

This is very common in Swahili. For example:

• nyumba = house
• nyumbani = at home / in the house

Not here. The idea of in/at is already included in gerezani because of the -ni ending.

So English uses two pieces:

• in prison

But Swahili can express that as one word:

• gerezani

Swahili can also use separate location words such as katika, but with many nouns, the -ni form is very natural and common.

Mfungwa is a singular noun referring to a person, and it belongs to the m-/wa- noun class pair for humans.

• singular: mfungwa = prisoner
• plural: wafungwa = prisoners

This matters because other words in the sentence often agree with that noun class.

You would change both the noun and the locative is/are form:

• Mfungwa yuko gerezani = The prisoner is in prison
• Wafungwa wako gerezani = The prisoners are in prison

Changes:

• mfungwa → wafungwa
• yuko → wako

No. Mfungwa is not specifically masculine.

It can refer to:

• a male prisoner
• a female prisoner

Swahili does not have grammatical gender like many European languages do, so the same noun can often refer to either sex unless something else makes it specific.

Swahili normally does not use articles like English a/an/the.

So mfungwa can mean:

• a prisoner
• the prisoner

The exact interpretation usually comes from context.

That is very normal in Swahili, and learners often need time to get used to not seeing articles.

Yuko tells you the subject is third person singular, referring here to he/she or the person being talked about.

In this sentence, it matches mfungwa, which is singular.

A useful comparison:

• yuko = he/she is
• wako = they are

So the form of yuko helps confirm that mfungwa is one person, not several.

The normal order here is:

Subject + location verb + place

So:

• Mfungwa = subject
• yuko = is located
• gerezani = in prison

This is a very natural and standard order in Swahili.

A simple pronunciation guide is:

• Mfungwa ≈ m-FOONG-gwa
• yuko ≈ YOO-ko
• gerezani ≈ geh-reh-ZAH-nee

A few helpful points:

• g is always hard, like in go
• vowels are usually clear and pure:
  a, e, i, o, u
• stress in Swahili is usually on the second-to-last syllable, so:
  • mfuNGWA
  • YUko
  • gereZAni

Normally, no. You would usually need the locative form:

• gerezani = in/at prison

Without -ni, gereza is just the noun prison, not the location expression in prison.

So Mfungwa yuko gerezani is the natural form.