Learn Hardcore Swahili: Kila mara ninapokosea, najaribu tena bila kukata tamaa. - Whenever I make a mistake, I try again without giving up.

-po- (in ninapokosea) often feels more neutral “when(ever)”, focusing on the time/occasion.
-ki- (in nikikosea) often feels more conditional “if/when this happens”, focusing slightly more on the condition–result idea.

In many everyday contexts, speakers use them almost interchangeably. Your original ninapokosea is very idiomatic with kila mara.

Why is it najaribu and not ninajaribu? Are both forms correct?

Both najaribu and ninajaribu are used and understood.

The underlying structure is:

ni- (I) + -na- (present) + jaribu (try)
→ ni-na-jaribu

In speech and informal writing, ninajaribu often gets shortened to najaribu. You will hear both, but:

najaribu – very common in speech, also fine in writing.
ninajaribu – a bit more “spelled out” and sometimes considered slightly more formal/clear.

In your sentence, najaribu is completely natural.

Is the comma between the two parts required in Swahili, and does it show a “when X, I Y” relationship?

The comma is not grammatically required, but it reflects the natural pause:

Kila mara ninapokosea, najaribu tena…

The important thing for the “when X, I Y” meaning is the structure:

[clause with -po- or -ki-], [main clause]

So even without the comma:

Kila mara ninapokosea najaribu tena bila kukata tamaa.

it is still understood as:

Whenever I make a mistake, I try again without losing hope.

How does bila work with verbs? Why is it bila kukata tamaa and not something like bila sikati tamaa?

bila means without.

When bila is followed by a verb, Swahili normally uses the infinitive (ku- form):

bila kula – without eating
bila kusema – without speaking
bila kukata tamaa – without losing hope

So the pattern is:

bila + ku- + verb (+ object if needed)

Using a finite verb like sikati (I don’t cut) after bila would be ungrammatical here. bila wants something more like a verbal noun: without cutting, without losing etc., which is what ku- forms give.

What does kukata tamaa literally mean, and is it a fixed expression?

Literally:

kata = cut
tamaa = desire / hope / longing

So kukata tamaa is literally to cut desire/hope, and idiomatically it means:

to lose hope, to give up, to become discouraged

Yes, kukata tamaa is a very common fixed expression. You will hear:

Usikate tamaa. – Don’t give up / Don’t lose hope.
Alikata tamaa kabisa. – He/She completely gave up.

In your sentence, bila kukata tamaa is a standard, idiomatic way to say without giving up / without losing hope.

Could I say bila tamaa or use matumaini instead, since they all relate to “hope”?

You could say other things, but the nuance changes:

bila tamaa – literally without desire/hope, but on its own this doesn’t mean without giving up; it sounds more like without any desire/hope at all.
matumaini = hope (more neutral word for hope than tamaa, which can also mean desire/greed).

Possible alternatives:

bila kupoteza matumaini – without losing hope (clear, but a bit more formal/less idiomatic than bila kukata tamaa).
bila kukata moyo – literally without cutting the heart, another idiom = without losing heart / without being discouraged.

bila kukata tamaa is the most common fixed phrase for without giving up / without losing hope.

Is the word order fixed, or can I move parts around, like putting kila mara later?

You can move things around somewhat, but keep the two main parts:

The “when/whenever” clause
The main clause

Some natural variants:

Najaribu tena bila kukata tamaa kila mara ninapokosea.
→ I try again without giving up every time I make a mistake.
Kila ninapokosea, najaribu tena bila kukata tamaa.
(dropping mara; still correct)

What you generally don’t do is split kila mara away from its verb in a confusing way, like:

✗ Ninapokosea kila mara, najaribu tena…
(grammatically possible but sounds odd, like “I make mistakes every time, I try again…”)

Keeping kila mara near ninapokosea or clearly modifying the whole clause is best.

What tense/aspect do ninapokosea and najaribu express here: past, present, or a general habit?

Here they express a general, habitual situation:

ninapokosea – whenever I make a mistake (in general)
najaribu tena – I try again (as a rule / by habit)

In Swahili, the -na- tense often covers:

present continuous (right now):
Ninakula. – I am eating.
general/habitual present:
Kila siku ninakula. – I eat every day.

With kila mara and -po-, the meaning becomes clearly habitual.

To talk about the past, you would use -li-:

Kila mara nilipokosea, nilijaribu tena bila kukata tamaa.
– Every time I made a mistake, I tried again without giving up.

Why is it kukata after bila, but just jaribu / kosea in the other verbs? What role does ku- play?

ku- here marks the infinitive form of the verb:

kata (verb root) → kukata (to cut)
sema → kusema (to speak)
imba → kuimba (to sing)

After bila, Swahili prefers the infinitive:

bila kula – without eating
bila kuzungumza – without speaking
bila kukata tamaa – without giving up

In ninapokosea and najaribu, the verbs are finite (they already have subject and tense markers), so you don’t add ku-:

ni-na-po-kosea – I-PRS-when-make a mistake
na-jaribu – I.PRS-try

Finite verb = no ku-.
Infinitive (as a kind of verbal noun, e.g. after bila) = ku- + verb.

Do I need to say mimi (I) in this sentence, like Kila mara mimi ninapokosea…?

You don’t need mimi here. The subject “I” is already expressed by the prefix ni- in ninapokosea and najaribu.

You might add mimi for emphasis or contrast:

Kila mara mimi ninapokosea, najaribu tena bila kukata tamaa.
→ As for me, whenever I make a mistake, I try again… (contrast with others who might give up)

But in neutral, everyday speech, the natural version is exactly what you have:

Kila mara ninapokosea, najaribu tena bila kukata tamaa.