Mamalia huyu anaishi kwenye maji.

Huyu means this for a singular animate noun, such as a person or animal.

In Swahili, words like this, that, adjectives, and many other modifiers usually come after the noun, not before it. So:

• mamalia huyu = this mammal

not something like this mammal word-for-word in English order.

Huyu also shows noun-class agreement. Because mamalia here refers to one living creature, Swahili uses the singular animate agreement pattern.

It can be either singular or plural depending on context.

Mamalia is a loanword, and in actual use the form often stays the same in singular and plural. The other words in the sentence tell you the number:

• mamalia huyu anaishi = singular
• mamalia hawa wanaishi = plural

So in your sentence, you know it is singular because of:

• huyu = this (singular animate)
• anaishi = he/she/it lives for a singular animate subject

Anaishi can be broken down like this:

• a- = subject marker for he/she or a singular animate noun
• -na- = present tense
• -ishi = verb stem meaning live

So:

• a-na-ishi = he/she/it is living / lives

The dictionary form of the verb is kuishi = to live.

Because Swahili usually packs subject and tense information into the verb itself.

In English, you say:

• it is living
• it lives

In Swahili, the verb already includes that information:

• anaishi

So you do not need a separate word for is here.

Kwenye is a very common locative word that can often mean:

• in
• on
• at

The exact English translation depends on context.

In this sentence, kwenye maji is understood as in the water.

So kwenye is doing the job that an English preposition would do.

Swahili often expresses location with locative forms such as kwenye, rather than using prepositions exactly the way English does.

So:

• kwenye maji = in the water

This is a very normal way to say it.

You may also meet other ways to express a similar idea, such as:

• majini = in the water / in waters
• katika maji = in water
• ndani ya maji = inside the water

But kwenye maji is completely natural and common.

Swahili does not usually have separate words for a, an, or the like English does.

So maji can mean:

• water
• the water

The context tells you which English translation fits best.

That is why there is no extra article word in the sentence.

Maji is usually treated as a mass noun meaning water, not as a countable singular noun like a water in normal English.

Historically and grammatically, it behaves like a noun in a plural-looking class, but for a learner the most useful thing is:

• maji = water
• it does not usually change form the way an English singular/plural pair would

So in this sentence, you can simply understand maji as water.

You would change the agreement words:

• Mamalia hawa wanaishi kwenye maji.

Breakdown:

• hawa = these (plural animate)
• wanaishi = they live / are living

So the pattern changes from singular animate:

• huyu anaishi

to plural animate:

• hawa wanaishi

In standard Swahili, mamalia huyu is the normal order for this mammal.

Swahili usually places the noun first and the demonstrative after it:

• mtu huyu = this person
• mnyama huyu = this animal
• mamalia huyu = this mammal

So Mamalia huyu is the form a learner should use.