Learn Hardcore Swahili: Watoto saba wanacheza mpira uwanjani. - Seven children are playing ball on the field.

So the plural of mtoto is watoto, not mtotos. This irregular-looking change is normal in Swahili: the prefix m- changes to wa- for this noun class.

Why is saba (seven) after watoto, when in English we say "seven children"?

In Swahili, most adjectives and numerals normally come after the noun they describe:

watoto saba = children seven
vitabu vitatu = books three
mtu mmoja = person one

So watoto saba is the normal order. Putting saba before (saba watoto) is not grammatical in standard Swahili.

Can saba change form to agree with watoto, like two children → watoto wawili?

No. Only the numbers 1–5 usually show agreement with the noun class:

1: mmoja / mmoja / mmoja (with different class prefixes)
2: -wili → wawili, wawili, wawili, etc.
3: -tatu
4: -nne
5: -tano

For people (class 1/2) you get:

mtoto mmoja – one child
watoto wawili – two children
watoto watatu – three children

But numbers 6 and above (sita, saba, nane, tisa, kumi, …) normally do not change:

watoto sita – six children
watoto saba – seven children

So saba stays saba; forms like wasaba are wrong.

What exactly is going on inside the verb wanacheza? Where is the "are" part?

Swahili usually builds "are" into the verb instead of using a separate word.

wanacheza breaks down like this:

wa- = subject prefix for they (here: they = children, class 2)
-na- = present tense / present continuous marker
-cheza = verb root cheza (to play)

So:

wana- (they + present) + -cheza (play) → wanacheza = "they are playing"

There is no separate word for are; it is expressed by the -na- in the verb.

Could you write wana cheza as two words, like "wana cheza"?

No. In standard writing, the subject prefix, the tense marker, and the verb root are written together as one word:

wanacheza, not wana cheza
anacheza, not ana cheza
walicheza (they played), not wali cheza

Native speakers may pause in speech, but in writing, it’s one word.

Does mpira just mean "ball," or does it mean "football/soccer"?

mpira can mean:

A physical ball (any kind of ball, depending on context)
The game of football/soccer, especially in East Africa, where football is very popular

So:

wanacheza mpira can mean:
- they are playing with a ball, or
- they are playing football/soccer

If you want to be more specific about the sport:

mpira wa kikapu – basketball
mpira wa miguu – football/soccer
mpira wa mikono – handball

In casual speech, mpira alone often implies football/soccer.

If I want to say "playing with the ball," should I add a preposition, like na?

Yes. To emphasize that they are playing with a ball (not necessarily playing football as a sport), you can say:

Watoto saba wanacheza na mpira uwanjani.
= Seven children are playing with a ball in the field.

The na here means with.

Without na, wanacheza mpira is more easily understood as playing the game of ball/football, depending on context.

What does the -ni at the end of uwanjani do?

The -ni suffix is a locative ending. It usually means in, on, or at a place.

uwanja = a field / ground (e.g. sports field)
uwanjani = in/on the field, at the field

Other examples:

nyumba = house → nyumbani = at home
shule = school → shuleni = at school
kanisa = church → kanisani = at church

You can often choose between:

uwanjani and
kwenye uwanja = in the field / on the field

Both are correct; uwanjani is more compact.

Is the definite article "the" included in words like uwanjani or mpira?

Swahili has no separate words for "a" or "the". Definiteness is understood from context, not grammar.

So uwanjani can mean:

in a field
in the field

Similarly, mpira can be a ball or the ball, depending on what has been mentioned or is obvious in the conversation.

If you need to be extra clear (for example, that field over there), you add demonstratives:

uwanja ule – that field
mpira ule – that ball

If I remove saba, do I just say Watoto wanacheza mpira uwanjani for "Children are playing ball in the field"?

Yes. Simply drop saba:

Watoto wanacheza mpira uwanjani.
= Children are playing ball in the field. / The children are playing ball in the field.

Everything else stays the same.

How do I say "The seven children are playing ball in the field" (emphasizing specific children)?

You can add a demonstrative to show you mean specific, known children:

Common patterns:

Watoto wale saba wanacheza mpira uwanjani.
= Those seven children are playing ball in the field.
Watoto hao saba wanacheza mpira uwanjani.
= Those seven children (previously mentioned) are playing ball in the field.

wale – those (over there)
hao – those (already mentioned / near you in conversation)

Both make the seven children clear in context.

How would I change this sentence to the past or future tense?

You change the tense marker in the verb (the part between the subject prefix and the verb root).

Present (given sentence):

wana-
- -cheza = wanacheza → they are playing

Past:

wali-
- -cheza = walicheza → they played / they were playing

Watoto saba walicheza mpira uwanjani.
Seven children played ball in the field.

Future:

wata-
- -cheza = watacheza → they will play

Watoto saba watacheza mpira uwanjani.
Seven children will play ball in the field.

Why is the verb wanacheza and not something with wa- only, like wacheza?

In a normal, finite verb, Swahili usually needs three pieces:

Subject prefix – here wa- (they, children)
Tense/aspect marker – here -na- (present)
Verb root – cheza (play)

So: wa- + -na- + cheza → wanacheza

wacheza without a tense marker would usually be something different, for example the subjunctive:

wacheze (notice the final -e) = that they play / let them play

So in straightforward statements about now, you want wana- + root → wanacheza.

Can I move uwanjani earlier in the sentence, like in English "In the field, seven children are playing ball"?

You can move it for emphasis, but the neutral order is:

Subject + (number) + Verb + Object + Place
Watoto saba wanacheza mpira uwanjani.

Other orders are possible in speech for emphasis or style, for example:

Uwanjani, watoto saba wanacheza mpira.
= In the field, seven children are playing ball.

This is acceptable and sounds like you are setting the scene (In the field...). But the original order is the most typical.