Usimkaribie mbwa huyo mgonjwa.

It is a negative command addressed to one person.

The beginning usi- tells you that the speaker is saying don't ... to a single listener.

So the sentence is not a plain statement; it is an instruction or warning: Don't approach that sick dog.

If you were speaking to more than one person, you would usually use msi- instead.

A useful breakdown is:

• usi- = don't you... / negative command to one person
• -m- = him/her/it object marker for a singular animate noun
• karibi- = verb stem from karibia (approach, go near)
• -e = the ending used here in this negative command form

So usimkaribie means roughly: don't approach him/it

Here, him/it refers to mbwa huyo mgonjwa.

The -m- is an object marker. It stands for the thing being approached: the dog.

In Swahili, when the object is a specific animate being—a person or animal—it is very common to include an object marker in the verb.

So:

• usimkaribie = don't approach it/him
• mbwa huyo mgonjwa = that sick dog

The verb and the full noun phrase both point to the same object.

Because Swahili often uses object marking together with the full noun, especially when the object is:

• specific
• known
• animate

That is very natural here, since the speaker means that particular sick dog.

You may sometimes hear a version without the object marker, but with a specific animal like this, the marked version is very normal and idiomatic.

Good question. Mbwa is one of those nouns whose form does not show m-/wa-, but because it refers to an animate being, Swahili often gives it animate agreement.

That is why you get things like:

• -m- object marker
• huyo demonstrative
• mgonjwa with m- agreement

So even though the noun itself is mbwa, its agreement often behaves like a singular animate noun.

This is very common with animals in Swahili.

Because negative commands in Swahili usually use the -e ending rather than the ordinary final -a.

So the basic verb is:

• karibia = approach / come near

But in this kind of sentence, it becomes:

• usimkaribie = don't approach him/it

This -e ending is a very common feature of command-like and subjunctive-style forms in Swahili.

Huyo means that one for a singular animate noun.

With animate singular nouns, the common set is:

• huyu = this one
• huyo = that one
• yule = that one over there / that one farther away

So:

• mbwa huyu = this dog
• mbwa huyo = that dog
• mbwa yule = that dog over there

In your sentence, huyo points to a specific dog the speaker has in mind.

Because in Swahili, modifiers usually come after the noun.

So it is normal to have:

• noun first: mbwa
• then words describing or identifying it: huyo, mgonjwa

That is one of the big differences from English, where we usually put adjectives before the noun:

• English: that sick dog
• Swahili: mbwa huyo mgonjwa

The exact order of modifiers can vary in some contexts, but the important thing for a learner is: the noun usually comes first.

Here it functions as an adjective-like modifier meaning sick / ill.

So in this sentence:

• mbwa ... mgonjwa = a sick dog

You may also see mgonjwa used as a noun, especially for a sick person / patient, depending on context. Swahili often allows the same form to work as a noun or as a modifier.

Here, because it follows mbwa, it is clearly describing the dog.

Yes. They are related in meaning.

• karibu often means near, and it is also used in expressions like welcome
• karibia is the verb come near / approach

So the sentence uses the verb form: usimkaribie = don't go near it/him

That is different from the everyday expression karibu! meaning welcome!

If you are speaking to more than one person, change usi- to msi-:

• Msimkaribie mbwa huyo mgonjwa.
• Don't approach that sick dog. said to several people

If there is more than one dog, the noun mbwa usually stays the same, but the agreement changes:

• Usiwakaribie mbwa hao wagonjwa.
• Don't approach those sick dogs. said to one person

Breakdown of the plural object part:

• wa- = plural animate object marker
• hao = those
• wagonjwa = sick plural

And if both are plural—several listeners and several dogs:

• Msiwakaribie mbwa hao wagonjwa.