Learn Hardcore Swahili: Mazoea ya kuandika shajara kila usiku ni muhimu kwetu. - The habit of writing a diary every night is important for us.

Mazoea comes from the verb kuzoea (to get used to / to be accustomed to).
As a noun, mazoea means things like habits, customs, usual practice, routine, familiarity.

Grammatically:

It belongs to noun class 5/6 (singular zoea, plural mazoea), but in real usage people almost always use the plural mazoea.
Often it behaves like an uncountable noun: mazoea ya kuandika shajara = “the habit / practice of writing a journal”, not really “habits” in the plural sense.

So in this sentence, you can understand Mazoea as “the habit” or “the practice” in a general way.

Why do we use ya in Mazoea ya kuandika shajara? What does ya do?

Ya is the possessive/associative connector meaning “of” or “’s”, and it agrees with the noun class of mazoea.

Mazoea is class 6, and the class‑6 possessive marker is ya.
So mazoea ya kuandika shajara literally means “habits of writing a journal” or “the habit of writing a journal”.

If the head noun were in a different class, ya would change, for example:

tabia (class 9/10) → tabia ya kuandika shajara
zoea (class 5) → zoea la kuandika shajara

Here, ya simply links mazoea to the action kuandika shajara.

What is kuandika here – is it a verb or a noun?

Kuandika is the infinitive form of the verb -andika (to write).

In Swahili, the infinitive ku- + verb often behaves like an English -ing noun:

kuandika ≈ “writing”
mazoea ya kuandika shajara ≈ “the habit of writing a journal”

So grammatically it’s a verbal noun: it looks like a verb, but in this position it functions like a noun (“writing” as an activity).

How should I understand the structure mazoea ya kuandika shajara? What is modifying what?

You can group it like this:

mazoea (habit / practice)
→ ya (of)
→ kuandika shajara (writing a journal)

So the whole chunk ya kuandika shajara is attached to mazoea and tells you what kind of habit it is:

mazoea ya [kuandika shajara] = “the habit of [writing a journal]”.

Inside that, kuandika is the verb “to write”, and shajara is its object (“journal/diary”).

What does shajara mean, and is it the usual word for “diary” or “journal”?

Shajara is a noun meaning “journal, diary, log, record book” – especially a book in which you write regular entries.

Other possibilities you might meet:

diari – a more direct borrowing of “diary” (also used).
daftari la kumbukumbu – literally “notebook of records/memories”.

In everyday language, shajara is a good, natural word for a personal diary/journal, so in this sentence it’s perfectly standard.

How does kila usiku work? Why is usiku singular if the meaning is “every night”?

Kila means “each / every” and it always takes a singular noun:

kila usiku = every night
kila siku = every day
kila mtu = every person

So usiku is singular in form, but kila usiku is understood as a repeated action: “every night”.
That’s why the sentence means “the habit of writing a journal every night”.

What is ni doing in … ni muhimu kwetu? Is it a verb?

Yes, ni is the basic copula “to be” in Swahili.

It links the subject Mazoea ya kuandika shajara kila usiku to the adjective muhimu (“important”).
ni does not change for number: it’s the same for “is” and “are”.

So:

Mazoea … ni muhimu kwetu = “The habit … is important to us.”
Because mazoea acts as a general subject here, you can also feel it as “Such habits are important to us”, but Swahili still just uses ni.

Why isn’t muhimu changed to agree with mazoea? Don’t adjectives usually agree with the noun class?

Many Swahili adjectives do change with the noun class, e.g.:

kitu kikubwa (one big thing), vitu vikubwa (big things)

But muhimu is one of the adjectives (borrowed from Arabic) that is invariable: it keeps the same form with all noun classes:

jambo muhimu – an important matter
vitu muhimu – important things
mazoea muhimu – important habits

So mazoea … ni muhimu is completely correct; muhimu does not take a special plural or class form.

What exactly does kwetu mean, and why use it instead of sisi?

Kwetu is a prepositional form meaning roughly “to us / for us / with us / at our place”, depending on context.

It is made of:

kwa (a general preposition: to/at/with/by)
-etu (our)

So kwetu is “to/at/with us (our side/place)”. In this sentence:

ni muhimu kwetu = “it is important to us / for us”.

You would not say ni muhimu sisi here; you need the prepositional form kwetu to express the idea of “important for us”.

Could I say Kuandika shajara kila usiku ni muhimu kwetu instead? What difference does it make?

Yes, that sentence is also correct:

Kuandika shajara kila usiku ni muhimu kwetu = “Writing a journal every night is important to us.”

The difference is subtle:

Mazoea ya kuandika… highlights the habit / routine itself.
Kuandika… highlights the action of writing.

In English terms, it’s like:

“The habit of writing a journal every night is important to us.”
vs.
“Writing a journal every night is important to us.”

Both are grammatical and natural; the original just emphasizes habit a bit more.