Translation:The uncle always cleans the kitchen.
mau 1. vs. Always, steady, constant, ever, unceasing, permanent, stationary, continual, perpetual; to continue, persevere, preserve, endure, last; ...
I is used to mark object, direct or indirect. That's the case here.
But i is also used to mark a location. The difference between i and ma is that ma is mostly used for fixed things, and i is used for something or someone moving to somewhere. You can use i and ma, it's pretty the same thing, you can use i for fixed thing too. But don't use ma if you are talking about a moving thing. For instance: the statue is in the middle of the place -> in the translation, you can use ma. But you can also use i. The statue is not moving. I'm coming to your house -> in the translation, you use i. Ma is wrong here, because you are moving.
But with the background, you can guess that in this sentence, i is used to mark the direct object. The uncle always cleans what? The kitchen, so put an i before the kitchen.