"We are in the kitchen"
why doesn't this sentence need a prepostion like "ndani ya?" And why does "Mtoto anacheza ndani ya nyumbani" need one? I'm confused about when to use the preposition with locatives and when not to.
Mtoto anacheza nyumbani tells us the child plays at home. That could just as easily be at home in the garden as it could be at home in the kitchen. The ndani ya gives us more specific information, telling us the child is inside.
Tumo jikoni, however, contains that information in the first word, tumo, "we are in".
Consider these three sentences:
Tuko mijini -- we are in town
Tupo nyumbani -- we are at home
Tumo jikoni -- we are in the kitchen
Each has a different suffix, and with the addition of a subject prefix we get three locative verbs which are all shades of "to be (there)".
The first uses the locative suffix for an indefinite location, -ko. "We are in town (somewhere)".
The second uses the locative suffix for a definite location, -po. If we ditched the playing from mtoto anacheza nyumbani, we could say mtoto yupo
* nyumbani, "the child is at home".
The third uses the locative suffix for being inside a location, -mo. Once again, if we ditch the playing from the child sentence, we could have mtoto yumo nyumbani, "the child is in the house".
In mtoto anacheza ndani ya nyumbani, we have another verb instead of our locative one. Instead we have "to play". So we need to specify location elsewhere. This is achieved by the addition of the preposition ndani ya, "inside of".
I hope this was clear, although I have a feeling it isn't.
* The third person singular is irregular, in that a- become yu- when using locatives. All other subject prefixes remain the same.
Short answer, because the -ni fulfils the same functional role as a preposition, indicating that the word is meant as a location.
And the sentence given was Mtoto anacheza ndani ya
nyumba. - not "ndani ya nyumbani"
juryrigging's answer was very good, as usual, but I want to expand on it a bit. Basically jikoni doesn't really stress "inside" but merely "at" (or "to" or "from" of course) . You can emphasise the interiority by using class 18 words with a locative noun.
ni = I'm inside the/a kitchen.
mwangu = I'm cooking inside my kitchen.
... but if you don't have any words that show concord in the sentence, you can also use "ndani ya" to emphasise the interiority another way.
ndani ya jiko. = I'm cooking inside the/a kitchen.
You could also simply say:
jikoni = I'm cooking in/at the kitchen.
... but that's not really specifically giving the interiority any emphasis ... maybe your kitchen is just an open cooking area and you're kind of next to it, stirring the pot ... if you really want to say you're contained within four walls, in the internal space of the kitchen, you can either use jikoni plus class 18 concord or "ndani ya jiko" ... I think they're more or less equivalent.