Learn Hardcore Swahili: Nyumba zao ni safi kila siku. - Their houses are clean every day.

Class 9/10 (like nyumba, siku, ndege): zao
- nyumba zao – their houses
- siku zao – their days
Class 3/4 or 5/6 (e.g. mti / miti, jicho / macho): yao
- miti yao – their trees
- macho yao – their eyes

Since nyumba here is plural in class 9/10, the correct possessive is zao, not yao.

Is ni the verb to be in this sentence? When do we use ni?

Yes. In Nyumba zao ni safi kila siku, ni is the present tense of to be (copula), linking the subject to a description:

Nyumba zao (subject)
ni (are)
safi (clean)

You normally use ni in the present when:

Linking a noun to a noun:
- Yeye ni mwalimu. – He/She is a teacher.
Linking a noun to an adjective:
- Chakula hiki ni kizuri. – This food is good.

You don’t normally leave it out here:
✗ Nyumba zao safi kila siku is not standard.

Could I say Nyumba zao ziko safi kila siku instead of Nyumba zao ni safi kila siku?

You can, and it’s understandable and used, but there is a nuance.

ni safi – describes what they are like; a more general description
ziko safi – uses the locative verb -ko (here ziko) and often feels more like are (in a state of being) clean / are found clean

In many everyday contexts the difference is small:

Nyumba zao ni safi kila siku. – Their houses are (characteristically) clean every day.
Nyumba zao ziko safi kila siku. – Their houses are (always kept) in a clean state every day.

Both are acceptable.

What part of speech is safi? Does it change form to agree with nyumba?

Safi is an adjective meaning clean, and also pure / neat / nice depending on context.

It does not change form for noun class or number:

nyumba safi – a clean house / clean houses
mtu safi – a decent / good person
vitu safi – clean / good things

Unlike adjectives such as -zuri (good), -kubwa (big), which take different forms:

mtu mzuri, watu wazuri
nyumba kubwa, nyumba kubwa (same in 9/10)

Safi stays the same in all these cases.

Do adjectives like safi come before or after the noun in Swahili? Could I say safi nyumba zao?

Adjectives in Swahili almost always come after the noun they describe.

So you say:

nyumba safi – clean house / clean houses
mtoto mzuri – good / pretty child

Putting safi before the noun (safi nyumba zao) is not normal Swahili word order.

In everyday speech you may hear some set phrases or emphatic uses that sound a bit different, but the standard pattern is:

[Noun] + [Adjective]
nyumba safi, nyumba nzuri, gari kubwa, etc.

What does kila siku mean exactly? Is it every day or all day?

Kila siku means every day / each day.

Nyumba zao ni safi kila siku.
→ Their houses are clean every day (day after day).

It does not mean all day. For all day, you would use something like:

siku nzima – the whole day
mchana kutwa – all day long (during the daytime)

So:

Nyumba zao ni safi kila siku. – Their houses are clean every day.
Nyumba zao ni safi siku nzima. – Their houses are clean all day (long).

Does kila go with singular or plural nouns, and does it change form?

Kila means each / every, and it:

Always takes a singular noun form
Does not change its own form

Examples:

kila siku – every day
kila mtoto – every child
kila nyumba – every house
kila mtu – every person

In our sentence:

kila siku = every day (literally: each day)

Can I change the word order and say Kila siku nyumba zao ni safi? Is that still correct?

Yes, that is correct Swahili.

Both are fine:

Nyumba zao ni safi kila siku. – Neutral order; focus is on their houses.
Kila siku nyumba zao ni safi. – Fronts kila siku for emphasis, like Every day, their houses are clean.

Swahili allows some flexibility in moving time expressions (like kila siku) to the beginning of the sentence for emphasis or flow.

How would I say Our houses are clean every day and My house is clean every day using the same pattern?

You just change the possessive word:

Our houses are clean every day.
→ Nyumba zetu ni safi kila siku.
- zetu = our (for plural class 9/10)
My house is clean every day.
→ Nyumba yangu ni safi kila siku.
- yangu = my (for singular class 9)

You can form others the same way:

Nyumba yako ni safi kila siku. – Your house is clean every day.
Nyumba zenu ni safi kila siku. – Your houses are clean every day (you plural).

How do I make this negative: Their houses are not clean every day?

There are two common ways, depending on what you want to stress.

Using the negative si (negative “to be” in the present):

Nyumba zao si safi kila siku.
→ Their houses are not clean every day.
(Maybe some days they are, some days they are not.)

Using the negative of -ko (if you use ziko):

Nyumba zao haziko safi kila siku.
→ Their houses are not (always) in a clean state every day.

Both are understood; si safi is often the most straightforward for beginners.

Is there another common way to say every day in Swahili, and is it different from kila siku?

Yes. Another very common expression is siku zote, literally all the days.

Comparison:

kila siku – every day, each day (neutral)
siku zote – all the time / on all days, often feels a bit stronger, like always

Examples:

Nyumba zao ni safi kila siku. – Their houses are clean every day.
Nyumba zao ni safi siku zote. – Their houses are clean all the time / always clean.

In many contexts they overlap, but siku zote can sound a bit more absolute.