Mwamba uko karibu na bahari.

Because uko is a form of to be used for location.
In this sentence, it means something like is located or is in a place.

So:

• Mwamba uko karibu na bahari = The rock/cliff is near the sea

Compare:

• Mwamba ni mkubwa = The rock is big
  
  • here ni links the subject to a description
• Mwamba uko karibu na bahari = The rock is near the sea
  
  • here uko shows location

Swahili verbs must agree with the noun class of the subject.

Mwamba belongs to the m-/mi- noun class pair, specifically singular class 3, whose subject marker is u-.
That is why you get u-ko → uko.

A few useful comparisons:

• mwamba → uko
• miamba (rocks/cliffs, plural) → iko
• mtu (person) → yuko

So uko is there because it matches mwamba.

Mwamba can mean a rock, large rock, rocky outcrop, cliff, or sometimes reef, depending on context.

So even if the translation shows one English meaning, the Swahili word can cover a small range of related ideas. Context tells you which one sounds best.

Its plural is miamba.

Karibu na means near, close to, or nearby to.

It is very commonly used to show physical closeness:

• shule iko karibu na nyumba = the school is near the house
• mji uko karibu na ziwa = the town is near the lake

In this sentence, karibu na bahari means near the sea.

No. In this sentence, na is not and.

Although na often means and or with, in the expression karibu na, it works as part of the phrase meaning near to.

So:

• karibu na bahari = near the sea
• not near and the sea

Context is what tells you which meaning of na is intended.

Because Swahili normally does not use articles like a, an, and the.

So a sentence like Mwamba uko karibu na bahari can be understood as:

• The rock is near the sea
• A rock is near the sea
• sometimes even The cliff is near the ocean

The exact English article depends on context, not on a separate Swahili word.

It can mean sea or ocean.
The right English choice depends on context.

So bahari is a broad word for a large body of salt water. In some situations, sea sounds more natural in English; in others, ocean does.

Yes, it is the same form, but the meaning depends on context.

The basic idea behind karibu is something like near / come near / come closer. From that, it can develop different uses:

• karibu na = near
• Karibu! = Welcome! / Come in!
• karibu by itself can also mean nearby or close

So in your sentence, it is clearly the near meaning.

Normally, no. For physical location, ni is usually not the natural choice here.

Use uko for location:

• Mwamba uko karibu na bahari = correct, natural

Using ni would sound wrong or unnatural to most learners' ears in this kind of sentence.

A good rule for now is:

• use ni for identification or description
• use uko / iko / yuko, etc. for location

The plural of mwamba is miamba.

So the sentence becomes:

• Miamba iko karibu na bahari.

Notice two changes:

1. mwamba → miamba
2. uko → iko

That is because the verb agreement changes with the plural noun class.

A simple pronunciation guide is:

Mwamba uko karibu na bahari
mwam-ba oo-ko ka-REE-boo na ba-HA-ree

A few tips:

• Swahili vowels are usually clear and consistent:
  • a as in father
  • i as in machine
  • u as in flute
• Stress is usually on the second-to-last syllable of each word:
  • mwAmba
  • Uko
  • karIbu
  • bahAri

Swahili pronunciation is generally quite regular, which makes it easier than English once you learn the sound system.