Theluthi moja ya maji imebaki.

In Swahili, theluthi means a third / one-third as a fractional unit. Then moja means one.

So:

• theluthi moja = one third
• theluthi mbili = two thirds

This is different from English word-building, because English uses a special form like third, while Swahili often uses the fraction word first and then adds the number.

Because in this kind of fraction expression, the normal order is:

• fraction word + number

So you say:

• theluthi moja = one third
• theluthi mbili = two thirds

Putting moja first would sound wrong here. A learner may expect English-style order, but Swahili structures this expression differently.

-a is the connector meaning of in expressions like this. The form changes to match the noun before it.

Here the structure is:

• theluthi moja ya maji
• literally: one third of water

So ya is the correct of-linker here.

A very important point: the linker agrees with theluthi, the head noun of the phrase, not with maji.

Because the verb is agreeing with theluthi moja, not directly with maji.

The subject of the sentence is the whole phrase:

• theluthi moja ya maji = one third of the water

Swahili often makes the verb agree with the main noun in that subject phrase. Here that main noun is theluthi, so the verb uses i-:

• imebaki = it has remained / is left

If maji itself were the subject, you would normally expect:

• Maji yamebaki = The water remains / the water is left

But in your sentence, the grammatical focus is on one third, so imebaki fits.

Maji is a mass noun meaning water, but grammatically it usually behaves like a plural noun in Swahili.

That is why, when maji is the subject, you often get plural-style agreement:

• Maji yameisha = The water is finished
• Maji yamebaki = The water remains

This can feel strange to English speakers, because water is singular in English. In Swahili, though, maji often takes class 6 agreement.

In your sentence, however, the verb agrees with theluthi moja, not with maji.

Imebaki comes from the verb -baki, meaning remain, be left, or stay over.

It can be broken down like this:

• i- = subject marker agreeing with theluthi
• -me- = perfect marker, often giving the sense has
• -baki = remain / be left

So imebaki means something like:

• has remained
• is left
• remains

In natural English, this sentence is often translated as One third of the water is left or One third of the water remains.

Because moja is the correct form here.

English speakers often learn mmoja first, but one changes form depending on noun class. With theluthi, the correct form is moja.

So:

• theluthi moja = correct
• theluthi mmoja = not correct here

This is a noun-class agreement issue, which is very common in Swahili.

Usually, if you want to say one third, keeping moja is clearer and more complete:

• theluthi moja ya maji imebaki

If you leave out moja, people may still understand you, but it can sound less explicit. For learners, it is safest to keep moja when you mean one third.

Theluthi is a correct word for third as a fraction, but it can sound a bit formal depending on context.

You may also hear other ways of expressing the same idea, especially in explanatory speech, such as wording based on sehemu (part/portion). But theluthi moja ya maji imebaki is perfectly understandable and grammatically sound.

So the sentence is good Swahili, even if in everyday conversation some speakers might choose a simpler or more conversational phrasing.

A simple approximate pronunciation for an English speaker is:

theh-LOO-thee MO-jah yah MAH-jee ee-meh-BAH-kee

A few tips:

• th in theluthi is usually pronounced like the th in think
• j in moja sounds like the j in jam
• maji has a hard j sound too: MAH-jee
• stress is usually not as strong as in English, but saying theh-LOO-thee MO-jah yah MAH-jee ee-meh-BAH-kee will be understood well

The structure is:

• theluthi moja = one third
• ya maji = of the water / of water
• imebaki = remains / is left

So the full pattern is:

• [fraction] + [of + noun] + [verb]

That makes the sentence:

• Theluthi moja ya maji imebaki.
• One third of the water remains.

This is a useful model you can reuse with other nouns.