Theluthi moja ya watu walifika.

It means one third or a third.

• theluthi = third / a third
• moja = one

So theluthi moja ya watu means one third of the people.

Because mmoja is used with class 1 singular human nouns, such as mtu mmoja = one person.

Here, the word being counted is theluthi, not mtu, so Swahili uses moja, not mmoja. In other words, moja agrees correctly with theluthi.

Here ya means of.

It links theluthi moja with watu:

• theluthi moja ya watu = one third of the people

This ya is the connector used after a noun like theluthi.

Because the people are the ones who actually arrived.

In Swahili, with expressions like one third of the people, the verb often agrees with the noun after ya, especially when it refers to people. Since watu is a human plural noun, the verb uses the plural human subject prefix wa-:

• wa-lifika = they arrived

So walifika is natural here because the sentence is really talking about people arriving.

It breaks down like this:

• wa- = they / plural human subject
• -li- = past tense
• fika = arrive

So walifika literally means they arrived.

The singular is mtu.

• mtu = person
• watu = people

This is important because watu takes plural human agreement, which is why the verb begins with wa-.

Because Swahili does not have articles like a, an, or the.

So watu can mean people, the people, or sometimes some people, depending on context. In this sentence, English often uses the people because the group is understood from the situation.

Sometimes you may hear theluthi ya watu for a third of the people, but theluthi moja ya watu is clearer and more explicit, especially for learners.

Using moja makes it completely clear that the fraction is one third.

Yes, but the meaning changes slightly.

• walifika = they arrived / simple past
• wamefika = they have arrived / present result, often with the sense that they are now here

So the original sentence is specifically using the past tense.