Learn Hardcore Swahili: Nyumba mpya itaanza kujengwa wiki ijayo. - The new house will start being built next week.

Nyumba belongs to noun class 9 (often called the N-class).
The subject marker for class 9 is i-.
So with nyumba, you use i-, giving itaanza, imejengwa, ilianguka, etc.

If the subject were a class 1 noun like mtoto (child), you would use a-:
Mtoto ataanza… = “The child will start …”

Why is the verb kujengwa (to be built) and not kujenga (to build)?

In English, the house is being built (passive voice), not building something itself.

kujenga = to build (active; someone builds something)
kujengwa = to be built (passive; something is built by someone)

A house cannot actively build, so nyumba itaanza kujenga would literally mean “the house will start to build (something)”, which is wrong.
You need the passive form kujengwa: “will start to be built.”

How is kujengwa formed from kujenga?

Take the verb -jenga (build):

Root: -jeng-
Passive suffix: -w-
→ jengw-
Final vowel -a for the infinitive verb form
→ jengwa
Add ku- in front to make the infinitive “to …”
→ kujengwa = “to be built”

So, kujenga → “to build”
and kujengwa → “to be built.”

Why are there two verbs, itaanza and kujengwa, next to each other? Is that normal?

Yes, this is very normal in Swahili.
Verbs like kuanza (to start), kuendelea (to continue), kujaribu (to try), kupenda (to like) often come before another verb in the infinitive:

Nitaanza kusoma. = I will start to read / start reading.
Alijaribu kuimba. = He/she tried to sing.
Watapenda kuja. = They will like to come.

In your sentence:

itaanza = will start
kujengwa = to be built

So itaanza kujengwa = “will start to be built.”

Where is the word “will” in this sentence? I only see itaanza.

Swahili does not use a separate word for “will”.
The future is shown inside the verb by the tense marker -ta-.

anaanza = he/she/it is starting (present; -na-)
ataanza = he/she/it will start (future; -ta-)
itaanza = it will start (future; i- for class 9 + -ta- for future)

So itaanza already contains the idea of “will”; you don’t add any extra word.

Why do we say wiki ijayo and not something like “in the next week” with a preposition?

Swahili often uses a bare time expression at the end (or beginning) of the sentence without a preposition:

Kesho = tomorrow
Leo jioni = this evening
Jumatatu = on Monday
Wiki ijayo = next week

You do not need “in”, “on”, “at” most of the time.
So Nyumba mpya itaanza kujengwa wiki ijayo literally is “The new house will start to be built next week.”
If you really want a preposition, you could say katika wiki ijayo, but in everyday speech wiki ijayo alone is normal.

What does ijayo mean, and why does it come after wiki?

Ijayo basically means “that is coming / that is next.” It comes from a form related to kuja (to come).

wiki ijayo = the week that is coming → “next week”
mwezi ujao = the month that is coming → “next month”
mwaka ujao = the year that is coming → “next year”

The describing word (ijayo, ujao, etc.) comes after the noun, not before it.
So you say wiki ijayo, not ijayo wiki.

Why does mpya come after nyumba instead of before it, like “new house” in English?

In Swahili, most adjectives come after the noun they describe:

nyumba kubwa = big house
mtoto mzuri = good child
gari jipya = new car
nyumba mpya = new house

So the natural order is noun + adjective, not adjective + noun.
That is why we say nyumba mpya, literally “house new.”

Is nyumba mpya singular or plural? How would I say “new houses”?

Nyumba is special because:

It is the same form for both singular and plural.
It belongs to noun class 9/10, where many words do not change between singular and plural.

So:

nyumba mpya can mean “a new house” or “new houses”, depending on context.
If you need to be very clear, you can add a number or another clue:
- nyumba mpya moja = one new house
- nyumba mpya tatu = three new houses
- nyumba hizi mpya = these new houses
- nyumba hii mpya = this new house

Could I say Nyumba mpya itajengwa wiki ijayo instead? What’s the difference from itaanza kujengwa?

Yes, you can say Nyumba mpya itajengwa wiki ijayo.

itájengwa = it will be built (future passive of kujenga)

Difference in meaning:

itaanza kujengwa = will start to be built (focus on the beginning of the building process)
itajengwa = will be built (focus on the whole action; building will happen, may suggest completion, depending on context)

So your original sentence emphasizes the start of construction next week, not necessarily finishing it next week.

How can I say who will build the house, like “by the workers,” in this sentence?

You can add na + the agent after the passive verb:

Nyumba mpya itaanza kujengwa na mafundi wiki ijayo.
= The new house will start to be built by the builders next week.

Other examples:

Nyumba itajengwa na kampuni kubwa.
= The house will be built by a big company.
Daraja linajengwa na serikali.
= The bridge is being built by the government.

How would I change this sentence to the past or present instead of future?

Change the tense marker in itaanza:

Future (original):
Nyumba mpya itaanza kujengwa wiki ijayo.
= The new house will start to be built next week.
Present (is starting / starts):
Replace -ta- with -na-
→ Nyumba mpya inaaanza kujengwa wiki hii.
= The new house is starting to be built this week.
(Note the three a’s in inaaanza: i + na + anza)
Past (started):
Use -li- for simple past
→ Nyumba mpya ilianza kujengwa wiki iliyopita.
= The new house started to be built last week.

So the pattern is: subject marker + tense marker + verb root.