Learn Hardcore Swahili: Baba analinda nyumba yetu usiku.

a- = subject marker for he/she (3rd person singular, class 1, used for people like baba)
-na- = present tense marker (general present)
linda = verb root meaning to guard / protect

So analinda means he/she guards or he/she is guarding.

If a- already means he/she, why do we still say Baba before the verb?

In Swahili, the verb almost always carries a subject prefix like a-, even when the subject noun is stated.

Baba analinda... = Baba tells you who the person is.
a- on analinda is required grammatical agreement with that subject.

You can drop Baba if it is already clear from context, and just say Analinda nyumba yetu usiku, but you cannot drop a- from the verb.

How do I say this sentence in the negative?

To make the present tense negative for he/she, you:

Change the subject prefix a- to ha-.
Drop the -na- present marker.
Change the final -a of the verb root to -i.

So:

Positive: Baba analinda nyumba yetu usiku.
Negative: Baba halindi nyumba yetu usiku. = Father does not guard / is not guarding our house at night.

Where is the or a in this sentence? Why is there nothing before baba or nyumba?

Swahili does not use articles like the or a/an.

Baba can mean a father, the father, or my/our father depending on context.
nyumba yetu can mean our house or our home without saying the.

Definiteness is understood from context or can be made clearer with other words, for example:

baba mmoja – one father / a father
nyumba fulani – a certain house

How can I tell if nyumba is singular or plural here?

The noun nyumba has the same form in both singular and plural (class 9/10), so you look at agreement words around it.

For possessives with nyumba:

Singular: nyumba yangu / yako / yake / yetu / yenu / yao
Plural: nyumba zangu / zako / zake / zetu / zenu / zao

In our sentence we have nyumba yetu, not nyumba zetu, so it is singular: our house, not our houses.

Why is it nyumba yetu, not nyumba ya yetu? When do we use ya?

Yetu is already the possessive adjective (our) with the right agreement for nyumba (class 9 singular). It goes directly after the noun:

nyumba yetu – our house

You use a form like ya when the possessor is a noun phrase, not a pronoun:

nyumba ya baba – the house of (the) father / the father’s house
nyumba ya rafiki yangu – the house of my friend

So:

with a pronoun: nyumba yetu
with a noun: nyumba ya baba

Can I move usiku to another place in the sentence, like Baba analinda usiku nyumba yetu?

Time expressions like usiku (at night) are flexible, but some positions sound more natural than others.

Common, natural options:

Baba analinda nyumba yetu usiku. (neutral; very natural)
Usiku, baba analinda nyumba yetu. (emphasis on at night)

Baba analinda usiku nyumba yetu is grammatically understandable but sounds unnatural to most speakers; usually the object nyumba yetu comes right after the verb, and time expressions come at the end or at the very beginning.

Does Baba here usually mean my father or just a/the father?

Literally, baba just means father.

In real conversation, though, when someone says Baba without any possessive (like baba yangu), it often implies “my father” if they are talking about their own family.

Baba analinda nyumba yetu usiku. – in many contexts this will be understood as My father guards our house at night.
To be fully explicit: Baba yangu analinda nyumba yetu usiku. – My father guards our house at night.

In stories or descriptions about other people, Baba can also mean the father of a certain family previously mentioned.

Does analinda mean “guards” (habit) or “is guarding” (right now)?

The -na- tense in Swahili usually covers both:

habitual present: He guards our house at night (it’s his regular duty).
progressive present: He is guarding our house at night (this is happening now / these nights).

Swahili doesn’t strictly separate these two the way English does; context or extra words give the nuance:

Kila usiku, baba analinda nyumba yetu. – Every night, father guards our house.
Sasa baba analinda nyumba yetu usiku. – Right now, father is guarding our house at night.

How would I say “Our father guards the house at night” instead?

You need our to modify baba, not nyumba:

Baba yetu analinda nyumba usiku. – Our father guards the house at night.

Changes compared with the original:

Baba yetu – our father
nyumba without yetu – now just the house (context supplies the).

Possessive adjectives always come right after the noun they belong to:
baba yetu (our father), nyumba yetu (our house), rafiki yake (his friend).

What would change if I wanted to say “Our houses” instead of “Our house” here?

For the plural houses, the possessive changes form:

Singular: nyumba yetu – our house
Plural: nyumba zetu – our houses

So you could say, for example:

Baba analinda nyumba zetu usiku. – Father guards our houses at night.

Here z- in zetu shows agreement with the plural form of nyumba (class 10).