Mamba huyu ni hatari kwa watoto.

You know it is singular because of huyu.

• huyu = this one / this for a singular animate noun
• So mamba huyu means this crocodile
• If it were plural, you would normally say mamba hawa = these crocodiles

So even though mamba itself can look the same in singular and plural, the agreement word tells you the number.

Huyu is a demonstrative, meaning this or this one.

In this sentence:

• mamba huyu = this crocodile

Swahili often uses demonstratives after the noun rather than before it.

Because that is the normal Swahili pattern.

Swahili usually says:

• noun + demonstrative

So:

• mamba huyu = this crocodile
• mtoto huyu = this child
• watoto hawa = these children

For an English speaker, this can feel reversed, but it is very natural in Swahili.

That is because animals are often treated as animate for agreement, especially when talking about a specific living creature.

So with mamba, Swahili commonly uses:

• singular animate demonstrative: huyu
• plural animate demonstrative: hawa

That is why mamba huyu sounds natural for this crocodile.

Ni is the copula, the word used for is / are in sentences like this.

So:

• mamba huyu ni hatari = this crocodile is dangerous

A useful thing to remember is that ni does not change for singular or plural:

• mtoto ni... = the child is...
• watoto ni... = the children are...

In sentences like this, hatari works like dangerous in English.

So:

• ni hatari = is dangerous

You can also think of it historically as a noun related to danger, but for a learner, the most useful point is this:

• hatari does not change form here
• it stays hatari no matter what noun comes before it

For example:

• mamba huyu ni hatari
• mbwa huyu ni hatari
• watoto hawa ni hatari

Because hatari is normally used as an invariable predicate word in this kind of sentence.

Unlike some other words in Swahili, it does not have to change its form to match noun class or number here.

So you do not need something like an English-style ending change. You just keep hatari as it is.

Kwa watoto means for children or to children, depending on how you want to express it in English.

In this sentence, it tells you who is affected by the danger:

• hatari kwa watoto = dangerous to children / dangerous for children

So:

• Mamba huyu ni hatari kwa watoto.
• This crocodile is dangerous to children.

Watoto means children.

Its singular form is:

• mtoto = child
• watoto = children

This is a very common singular/plural pair in Swahili:

• m- in the singular
• wa- in the plural

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

Whether something is definite or indefinite usually comes from context.

In this sentence, huyu already makes it definite, because this points to a specific crocodile:

• mamba huyu = this crocodile

So Swahili does not need a separate word for the.

As a learner, you should keep ni here.

The standard sentence is:

• Mamba huyu ni hatari kwa watoto.

Without ni, the sentence sounds incomplete in standard Swahili. So the safest rule is:

• when saying X is Y in this kind of sentence, use ni

You would say:

• Mamba hawa ni hatari kwa watoto.

Changes:

• huyu → hawa for animate plural
• mamba stays mamba
• ni hatari kwa watoto stays the same

So:

• Mamba huyu ni hatari kwa watoto. = This crocodile is dangerous to children.
• Mamba hawa ni hatari kwa watoto. = These crocodiles are dangerous to children.