Mchuzi unakuwa mtamu ukiutia pilipili manga kidogo.

Because mchuzi belongs to the m-/mi- noun class in Swahili.

• singular: mchuzi
• plural: michuzi

In the singular, this noun class usually takes the subject marker u-. So:

• mchuzi unakuwa... = the sauce becomes/is becoming...

This is noun-class agreement, which is very important in Swahili. The verb has to match the noun class of the subject.

Swahili adjectives usually agree with the noun class of the noun they describe.

The adjective stem is -tamu = sweet / tasty / delicious.

Since mchuzi is in the singular m-/mi- class, the adjective takes the matching prefix m-:

• mchuzi mtamu = tasty sauce

So in the sentence:

• Mchuzi unakuwa mtamu...

both the verb and the adjective are agreeing with mchuzi.

Unakuwa comes from:

• u- = subject marker agreeing with mchuzi
• -na- = present tense
• -kuwa = be / become

In this sentence, unakuwa mtamu is best understood as becomes tasty or gets tasty.

So the idea is not just that the sauce is tasty, but that adding the pepper makes it turn out tasty.

Ukiutia can be broken down like this:

• u- = you (singular)
• -ki- = if / when
• -u- = it (object marker referring to mchuzi)
• -tia = put / add / place into

So ukiutia literally means something like:

• if/when you add it to it
• more naturally: if/when you add to it

In the full sentence, the thing being added is pilipili manga kidogo.

That -u- is an object marker referring back to mchuzi.

So Swahili is saying:

• uki-u-tia
• when/if you add to it

Here, it = the sauce.

English often does not need to show this inside the verb, but Swahili commonly does.

It can mean either, depending on context.

In many sentences, -ki- expresses a general condition:

• if
• when
• whenever

So here ukiutia pilipili manga kidogo can be understood as:

• if you add a little black pepper
• when you add a little black pepper
• whenever you add a little black pepper

All of these are close in meaning here.

Grammatically, yes: u- is the singular you.

But in sentences like this, Swahili often uses singular you in a general way, just like English does in sentences such as:

• If you add a little pepper, the sauce tastes better.

So it does not necessarily mean the speaker is talking to only one specific person. It can mean anyone / one / people in general.

If you wanted a plural you all, you could use mki- instead of uki-.

Pilipili manga means black pepper.

That is different from hot chili peppers. In Swahili:

• pilipili by itself can refer to pepper/chili
• pilipili manga specifically means black pepper

So in this sentence, it means the seasoning black pepper, not necessarily fresh chili.

Kidogo means a little / a small amount.

Here it modifies pilipili manga, so:

• pilipili manga kidogo = a little black pepper

In Swahili, words like kidogo often come after the noun they describe in this kind of phrase. So the order feels natural in Swahili even though English puts a little before the noun.

Yes. That word order is also natural.

Swahili often allows the conditional part to come first or second:

• Mchuzi unakuwa mtamu ukiutia pilipili manga kidogo.
• Ukiutia pilipili manga kidogo, mchuzi unakuwa mtamu.

Both mean the same thing. The difference is mainly about emphasis and flow.

A plural version with sauces would need plural agreement:

• Michuzi inakuwa mitamu ukiitia pilipili manga kidogo.

Changes:

• mchuzi → michuzi
• u- subject marker → i-
• mtamu → mitamu
• object marker inside ukiitia also changes to agree with michuzi

This shows how strongly Swahili grammar depends on noun-class agreement.

Yes. In this sentence, -tia is best translated naturally as add.

Depending on context, it can mean:

• put
• place
• apply
• add

Here, since the thing being put in is black pepper, add is the most natural English choice:

• when you add a little black pepper to it