Learn Hardcore Swahili: Jana mchana nilikuwa nikisoma kitabu changu wakati mvua ilikuwa ikinyesha. - Yesterday afternoon I was reading my book while it was raining.

Nilisoma = I read / I studied (a finished action in the past).
Nilikuwa nikisoma = I was reading / I was in the middle of reading (an ongoing action in the past).

In this sentence we want to show that:

the reading was in progress at a certain time in the past (yesterday afternoon)
and something else happened at the same time (it was raining)

So Swahili uses kuwa in the past (nilikuwa) + the -ki- form of the main verb (nikisoma) to express past continuous, similar to English was reading.

You could say Jana mchana nilisoma kitabu changu, but that would mean something more like Yesterday afternoon I read my book (finished action), not emphasizing that it was ongoing at that time.

What exactly is going on inside nilikuwa nikisoma grammatically?

You can break nilikuwa nikisoma down like this:

ni-li-kuwa
- ni- = I (subject marker, 1st person singular)
- -li- = past tense
- -kuwa = to be → nilikuwa = I was
ni-ki-soma
- ni- = I (subject marker again)
- -ki- = marker often used for an action that is ongoing, or backgrounded
- -soma = read

So nilikuwa nikisoma literally feels like I was I-ki-read, which functions as I was reading.

The repetition of ni- is normal and required: each verb form (even in the same clause) gets its own subject marker.

Why does it say mchana right after Jana? Is that like saying yesterday at noon / in the afternoon?

Yes. Jana mchana is a common time expression:

jana = yesterday
mchana = daytime / afternoon (roughly late morning to before evening)

Putting them together:

Jana mchana ≈ yesterday afternoon / yesterday during the day

No extra preposition (at, in, on) is needed. In Swahili you usually just stack time expressions like this:

Jana asubuhi – yesterday morning
Jana jioni – yesterday evening
Jana usiku – last night

So Jana mchana nilikuwa… = Yesterday afternoon I was…

Why is it kitabu changu and not kitabu yangu?

The possessive form in Swahili must agree with the noun class of the thing possessed.

kitabu (book) is in noun class 7 (Ki-/Vi- class).
The class-7 form of -angu (my) is changu, not yangu.

Some examples:

kitabu changu – my book
vitabu vyangu – my books (class 8, Vi- → vyangu)
mtoto wangu – my child (class 1, M- → wangu)
meza yangu – my table (class 9/10, N- → yangu)

So kitabu changu is just correct noun-class agreement; kitabu yangu would be incorrect.

Why do we also get this long form mvua ilikuwa ikinyesha for it was raining instead of just mvua ilinyesha?

Same idea as with nilikuwa nikisoma:

mvua ilinyesha = it rained (completed event)
mvua ilikuwa ikinyesha = it was raining (ongoing background action)

Since the English sentence is I was reading my book while it was raining, both actions are in progress at the same time. Swahili mirrors that with:

nilikuwa nikisoma – I was reading
mvua ilikuwa ikinyesha – it was raining

If you said Jana mchana nilisoma kitabu changu wakati mvua ilinyesha, it would sound more like two completed events, not clearly simultaneous ongoing actions.

What is the function of -ki- in nikisoma and ikinyesha?

In forms like nikisoma and ikinyesha, -ki- marks an action as ongoing / in progress, or often as a backgrounded action relative to something else.

Breakdown:

ni-ki-soma – I ki-read → (while) I am / was reading
i-ki-nyesha – it ki-rain → (while) it is / was raining

Common uses of -ki-:

With kuwa to make past continuous:
- Nilikuwa nikisoma. – I was reading.
In clauses meaning when / while:
- Nilimpigia simu akiwa kazini. – I called him while he was at work.

So in your sentence, -ki- helps show that the actions are ongoing, not single, finished events.

What does wakati do here, and is it the same as when or while in English?

Wakati means time, but in context it works like when or while introducing a time clause.

wakati mvua ilikuwa ikinyesha ≈ when it was raining / while it was raining

A few patterns with wakati:

wakati + clause:
- Wakati mvua ilinyesha, baridi iliingia. – When it rained, the cold came.
wakati wa + noun:
- Wakati wa mvua… – During the rainy season / during the rain.

In your sentence, wakati introduces the background situation in which the main action (reading) was taking place.

Could I put the wakati clause at the beginning, like in English: While it was raining, I was reading my book?

Yes. Swahili word order is flexible with time clauses. You can say:

Jana mchana nilikuwa nikisoma kitabu changu wakati mvua ilikuwa ikinyesha.
Jana mchana, wakati mvua ilikuwa ikinyesha, nilikuwa nikisoma kitabu changu.

Both are natural. Just keep all the verb agreements and tenses the same; you’re only moving the clause.

How is the subject I shown in nilikuwa and nikisoma? There is no separate I word.

Swahili normally does not use separate subject pronouns (like I, you, he) the way English does. Instead, the subject marker is attached to the verb:

ni- = I
u- = you (singular)
a- = he / she
tu- = we
m- = you (plural)
wa- = they

So:

nilikuwa = ni- (I) + -li- (past) + -kuwa (be) → I was
nikisoma = ni- (I) + -ki-
- -soma → (when/while) I was reading

You can add an independent pronoun like mimi for emphasis:

Mimi nilikuwa nikisoma… – I was reading… (emphasizing I), but it’s not required.

What exactly does mchana mean in terms of time of day? Is it closer to daytime or afternoon?

Mchana can mean both daytime and afternoon, depending on context.

Very roughly:

asubuhi – morning
mchana – late morning through afternoon (daytime as opposed to night)
jioni – late afternoon / early evening
usiku – night

In a sentence like Jana mchana nilikuwa nyumbani, an English speaker will usually understand it best as yesterday afternoon I was at home, though yesterday during the day is also possible.

Context usually makes it clear enough; there isn’t always a sharp line between daytime and afternoon in Swahili usage.

Could I simplify the sentence to something shorter but still natural?

Yes. Depending on how much detail you want, you could say:

Jana mchana nilisoma kitabu changu wakati mvua ilinyesha.
– Yesterday afternoon I read my book when it rained.
(Both actions more like completed events.)
Jana mchana nilikuwa nasoma kitabu changu wakati mvua ilinyesha.
– A mix: I was reading my book when it rained (one ongoing, one event).

Your original:

Jana mchana nilikuwa nikisoma kitabu changu wakati mvua ilikuwa ikinyesha.

is the most clearly past continuous + past continuous version, matching I was reading my book while it was raining very closely in aspect.