Learn Hardcore Swahili: Ninaandika msamiati mpya katika daftari la mazoezi kila siku. - I write new vocabulary in the exercise notebook every day.

msamiati = vocabulary (literally, a vocabulary list; noun, class 3/4 or m-/mi- type)
mpya = new (adjective; agrees with msamiati)
katika = in / inside / within
daftari = notebook / exercise book (noun, class 5/6: ji-/ma- type, but often without the ji- in the singular)
la = of (associative marker “of” for class 5 singular nouns like daftari)
mazoezi = exercises, practice (noun, class 6, ma- prefix)
kila = every / each
siku = day

Natural English: I write new vocabulary in the exercise notebook every day.

What exactly does Ninaandika mean, and how is it formed?

Ninaandika is a conjugated verb meaning I am writing / I write.

It is built from three parts:

ni- = subject prefix for I
-na- = present tense marker (present continuous or general present)
-andika = verb stem write

So structurally: ni + na + andika → ninaandika.

Use:

It can mean I am writing (right now).
With a time phrase like kila siku, it is often understood as I write (habitually).

So in the sentence, because of kila siku (every day), ninaandika is best read as a habitual action: I (regularly) write.

Do I need to say mimi as well, like Mimi ninaandika msamiati mpya…?

You do not need to. The subject prefix ni- in ninaandika already tells you the subject is I.

Ninaandika msamiati mpya… = I write new vocabulary… (normal, neutral)
Mimi ninaandika msamiati mpya… = I write new vocabulary… (with emphasis on I, e.g. contrasting with someone else)

So:

Use ninaandika alone in most cases.
Add mimi when you want to stress I (and not someone else).

Why is it msamiati mpya and not mpya msamiati? What is the normal word order for nouns and adjectives?

In Swahili, the adjective usually comes after the noun it modifies.

msamiati mpya = new vocabulary
- msamiati = vocabulary (noun)
- mpya = new (adjective)

Putting the adjective first, mpya msamiati, is not grammatical.

Also note agreement:

msamiati is in the m-/mi- class (class 3/4), singular.
The adjective -pya takes a class agreement prefix: m-, becoming mpya.
If it were plural (misamiati), it would still be mpya (the form mpya happens to be the same for many classes in singular and plural).

So the key points:

Noun first, adjective after: noun + adjective.
Adjective form agrees with the noun class.

What does daftari la mazoezi literally mean, and why is it la, not ya or cha?

Daftari la mazoezi literally means notebook of exercises or exercise notebook.

Structure:

daftari = notebook (head noun, class 5, singular)
la = associative “of” for class 5 singular
mazoezi = exercises / practice (noun, class 6)

In Swahili, the “of” word (called the associative marker) changes depending on the class and number of the first noun:

Class 1/2: wa (mtu wa kazi – person of work)
Class 3/4: wa / ya
Class 5/6: la / ya
Class 7/8: cha / vya, etc.

Since daftari is class 5 singular, its associative form is la:

daftari la mazoezi = notebook of exercises

You would use cha, ya, wa, etc. with different noun classes, but la is correct here.

What is the difference between katika daftari and other ways of saying in the notebook, like kwenye daftari or daftarini?

All of these can express in the notebook, but with slightly different styles/nuances:

katika daftari la mazoezi
- katika ≈ in / inside / within.
- Often a bit neutral/formal, common in written or careful Swahili.
kwenye daftari la mazoezi
- kwenye ≈ at / on / in (very common in everyday speech).
- Slightly more colloquial than katika, but widely used and accepted.
daftarini
- This uses the locative suffix -ni on daftari.
- daftarini = in the notebook / in a notebook.
- You could say: Ninaandika msamiati mpya daftarini kila siku.
- This is natural and common.

So:

Original: katika daftari la mazoezi – perfectly correct, somewhat neutral/formal.
Very common alternative: kwenye daftari la mazoezi.
Shorter, also good: daftarini (possibly daftarini la mazoezi).

How does kila siku work? Why is siku singular even though it means every day (plural in English)?

Kila siku means every day.

kila = each / every
siku = day (but siku is the same form in singular and plural)

Key points:

kila is always followed by a singular noun in form, even though the meaning is repeated (many days).
- kila siku = every day
- kila mwanafunzi = every student
- kila nchi = every country
siku itself does not change form between singular and plural:
- siku = day / days (context tells you which).

So kila siku is grammatically singular (every day one by one), but semantically it refers to something that happens on all days.

Could the word order be changed, for example to Kila siku ninaandika msamiati mpya katika daftari la mazoezi? Is that still correct?

Yes, that word order is also correct.

Both are fine:

Ninaandika msamiati mpya katika daftari la mazoezi kila siku.
Kila siku ninaandika msamiati mpya katika daftari la mazoezi.

Swahili word order is relatively flexible with time expressions like kila siku:

Putting kila siku at the end is very common.
Putting kila siku at the beginning can slightly emphasize the frequency/time: Every day, I write….

Other parts of the sentence (subject–verb–object–prepositional phrase) generally keep the same order, but adverbials like kila siku, leo, kesho, etc. can move around.

If I want to say I write new vocabulary in my exercise notebook every day, how do I add my?

You would add a possessive adjective that agrees with daftari (class 5). The class‑5 singular possessive for my is -angu → langu.

So:

daftari langu la mazoezi = my exercise notebook

Complete sentence:

Ninaandika msamiati mpya katika daftari langu la mazoezi kila siku.
= I write new vocabulary in my exercise notebook every day.

Pattern:

daftari langu = my notebook
daftari lake = his/her notebook
daftari letu = our notebook, etc.

Does Ninaandika msamiati mpya katika daftari la mazoezi kila siku sound more like I am writing or I write in English?

By itself, ninaandika can mean either:

I am writing (present continuous, right now)
I write (present habitual / general truth)

In this sentence, because of kila siku (every day), the meaning is clearly habitual:

I write new vocabulary in the exercise notebook every day.

If you wanted to make a stronger, clearly habitual form, especially in some varieties of Swahili, you might also hear:

Huandika msamiati mpya katika daftari la mazoezi kila siku.
(hu- is a special habitual marker)

But using ninaandika … kila siku is completely natural and widely used to express a regular habit.