Learn Hardcore Swahili: Watoto wanakaa sawasawa mezani jioni. - The children are sitting evenly at the table in the evening.

wa- = subject prefix for they / children (noun class 2, matching watoto)
-na- = present tense marker (often called the -na- tense)
kaa = verb root meaning sit / stay / remain / live (depending on context)

So wa + na + kaa → wanakaa = they sit / they are sitting / they stay, depending on context and extra time words like sasa (now), kila siku (every day), etc.

Could wanakaa here also mean they live? How do I know it means sit?

The verb kaa is flexible: it can mean sit, stay, or live (reside).

wanaishi Dar es Salaam – they live in Dar es Salaam
wanakaa Dar es Salaam – they live/stay in Dar es Salaam
wanakaa mezani – they sit/stay at the table

In your sentence, the presence of mezani (at the table) and jioni (in the evening) strongly suggests a temporary posture/activity, not residence. People don’t live “at the table in the evening”, so in normal context this is naturally understood as sit.

Is watoto here the children or just children? Why isn’t there a word for the?

Swahili does not have articles like the / a / an. A noun like watoto can be understood as:

the children
children
some children

The exact sense (definite vs indefinite) comes from context, not from a separate word. If the listener already knows which children you mean, they will hear watoto as the children. If not, it will feel more like children / some children in general.

What does sawasawa do in this sentence? Is it like an adverb, and can I replace it with vizuri?

Yes, sawasawa here functions like an adverb, describing how the children sit.

sawasawa can mean properly, neatly, evenly, in an orderly / correct way.
vizuri means well, nicely and is more general.

In this sentence:

wanakaa sawasawa mezani – they sit properly / in an orderly manner at the table
wanakaa vizuri mezani – they sit well / nicely at the table (also correct and natural)

Both are fine. Sawasawa adds a nuance of orderliness / straightness / all in line, whereas vizuri is just well in a broad sense.

Why is it mezani and not just meza? What does the -ni ending mean?

Mezani = meza (table) + -ni (locative suffix).

The suffix -ni often means in / at / on a place.

nyumba (house) → nyumbani – at home / in the house
shule (school) → shuleni – at school
meza (table) → mezani – at the table / on the table

So mezani already includes the idea of at / on, so you don’t need another preposition for that location.

Could I also say kwenye meza instead of mezani? Is there a difference?

Yes, you can say kwenye meza, and it is grammatically fine:

wanakaa mezani
wanakaa kwenye meza

Both can mean they sit at the table / on the table.

Subtle differences:

mezani is shorter and very common; it feels slightly more “built-in” and idiomatic.
kwenye meza is also natural; kwenye is a general preposition meaning in/at/on.

In most everyday speech, mezani is what you will hear more when talking about sitting at the table.

Why is there no preposition before jioni? In English we say in the evening.

Swahili often uses bare time nouns as time adverbs, without a preposition:

asubuhi – (in the) morning
mchana – (in the) afternoon / daytime
jioni – (in the) evening
usiku – (at) night

So jioni by itself means in the evening / this evening / evenings, depending on context. You normally do not say kwa jioni or katika jioni in this simple sense; just jioni is standard.

Does this mean they always do this every evening, or that they’re doing it this particular evening? How does Swahili show that?

With -na- (as in wanakaa), Swahili present tense is often ambiguous between:

present progressive: they are sitting (now)
general/habitual: they sit (as a habit)

In your sentence, adding jioni makes a habitual reading quite natural: they sit properly at the table in the evening / in the evenings.

If you want to make habit especially clear, you can use hu- (habitual):

Watoto hukaa sawasawa mezani jioni. – The children (usually) sit properly at the table in the evenings.

If you want right now, you can add sasa:

Watoto wanakaa sawasawa mezani sasa jioni. – The children are sitting properly at the table now this evening. (A bit wordy, but clear.)

Can I change the word order, like Jioni watoto wanakaa sawasawa mezani? Is that natural?

Yes, Swahili word order is fairly flexible for time and place phrases.

All of these are possible and natural, with slight emphasis differences:

Watoto wanakaa sawasawa mezani jioni. – neutral; focus on the action, then when.
Jioni watoto wanakaa sawasawa mezani. – emphasizes in the evening.
Watoto jioni wanakaa sawasawa mezani. – still okay, though less common.

What you should normally keep is:

subject near the beginning (here: Watoto)
verb right after subject (here: wanakaa)

Time and place phrases can move more freely around them.

If there is only one child, how would the sentence change?

For a single child:

Watoto → Mtoto (child)
wanakaa (they sit) → anakaa (he/she sits)

So you get:

Mtoto anakaa sawasawa mezani jioni. – The child sits properly at the table in the evening.

Everything else (sawasawa, mezani, jioni) stays the same.