Learn Hardcore Swahili: Kabla hatujaanza chakula cha jioni, mama hupenda kuhesabu watoto wote mezani. - Before we start dinner, mother likes to count all the children at the table.

So hatujaanza literally means we have not yet started.
In this sentence it appears inside a before clause: kabla hatujaanza chakula cha jioni = before we have started dinner / before we start dinner.

Why does the sentence say kabla hatujaanza and not kabla ya kuanza?

Swahili has two common patterns with kabla (before):

kabla
- full clause (usually negative perfect)
- Kabla hatujaanza chakula cha jioni …
- Literally: Before we have not yet started dinner …
- Idiomatic English: Before we start dinner …
kabla ya
- noun or infinitive
- Kabla ya kuanza chakula cha jioni, …
- Literally: Before starting dinner …

Both are correct here. The version in your sentence uses pattern 1, which is very common in conversation and writing when talking about things that must happen before some other event.

Is hatujaanza talking about the present, past, or future?

Grammatically, hatujaanza is present perfect negative (we have not yet started).
However, inside a kabla clause, the whole idea points to a time that is before a future event (starting dinner).

So:

hatujaanza by itself: we have not yet started (present result of a past non‑action).
kabla hatujaanza chakula cha jioni: before we start dinner / before we have started dinner (refers to a time that is earlier than the moment of starting).

What is the function of hu- in mama hupenda?

The prefix hu- on a verb marks a habitual or general action.

mama hupenda kuhesabu …
= Mom likes / tends to / usually likes to count … (habit, general truth)

Compare:

mama anapenda kuhesabu …
= also Mom likes to count …, but without strongly highlighting the habitual, rule‑like nature.

Both are correct, but hu- is a special marker used only for this kind of general, timeless habit.

Could we say mama anapenda kuhesabu instead of mama hupenda kuhesabu? Would it change the meaning?

Yes, you can say mama anapenda kuhesabu watoto wote mezani.

The difference is mainly in nuance:

mama hupenda kuhesabu …
Emphasizes a regular habit, almost like a rule in the household.
mama anapenda kuhesabu …
States that she likes doing it; context will tell you if it is habitual.

In most everyday situations, both will be understood almost the same, and both are correct.

How does chakula cha jioni work grammatically?

Chakula cha jioni is a possessive or of‑phrase:

chakula = food / meal (noun class 7)
cha = of for class 7 nouns (possessive concord for class 7)
jioni = evening

Literally it is food of evening, which corresponds to dinner in English.
Other similar expressions:

chakula cha asubuhi = breakfast
chakula cha mchana = lunch

Why do we use cha in chakula cha jioni and not ya?

The form of of in Swahili agrees with the noun class of the head noun.

chakula is in noun class 7 (ki-/vi- class in the singular is often ki-/chi- or ch-, but the possessive uses the class marker cha).

So:

chakula cha jioni = food of evening
If the head noun were class 9 or 10, we would use ya, for example:
- siku ya jioni = the day of the evening

So cha is chosen purely because chakula is in class 7.

What exactly does kuhesabu mean, and what is the role of ku-?

Kuhesabu means to count (to count numbers or items).

ku- at the beginning marks the infinitive form of the verb (equivalent to to in English to count).
hesabu is the verb stem meaning count; it is also a related noun meaning number / calculation in other contexts.

So mama hupenda kuhesabu watoto wote = Mom likes to count all the children.

What is the difference between watoto and watoto wote?

watoto = children
wote = all (for people in plural, class 2 agreement)

Putting them together:

watoto wote = all the children

The adjective wote comes after the noun here, which is the normal order with full nouns:

watoto wote = all the children
marafiki wote = all the friends

Can wote ever come before the noun, like wote watoto?

With full nouns, the natural and standard order is:

noun + wote → watoto wote, walimu wote, marafiki wote

Wote usually comes before only with pronouns or with some special emphatic orders:

wote wao = all of them
wote sisi = all of us

So for your sentence, watoto wote is the normal and expected word order.

What does mezani literally mean, and why is there -ni at the end?

Mezani comes from:

meza = table
-ni = a locative suffix meaning in / at / on (location)

So mezani literally means at the table / on the table.

This -ni locative is very common:

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

Could we say kwa meza instead of mezani?

You might hear kwa meza in some contexts, and it can mean by the table / at the table, but:

mezani is the most natural and idiomatic way to say at the table in this kind of sentence.
kwa often has a sense of by / at somebody’s place / via, or marks a general location, while -ni on the noun directly makes a clear, fixed place.

So in your sentence, watoto wote mezani is the best, most standard phrasing.

Why does Swahili use a negative form hatujaanza after kabla, when English says before we start (positive)?

Swahili commonly uses a negative perfect after kabla to express not having yet done something at that earlier time:

kabla sijaondoka = before I (have) left / before I leave
kabla hatujaanza chakula cha jioni = before we (have) started dinner / before we start dinner

English normally keeps the verb positive (before we start), but Swahili focuses on the idea that, at that time, the action has not yet happened.

So the negative perfect -ja- with kabla is a very typical and natural structure in Swahili.