So an active sentence describes someone doing something
Rangi is reading a book
The mother is baking a cake
I am eating a potato
A passive sentence describes something being done to someone or something
The book is being read by Rangi
The cake is being baked by the mother
The potato is being eaten by me.
In te reo Māori we mark the object of the sentence with i
Kei te pānui a Rangi *i* te pukapuka
Rangi is reading the book
Kei te tunu te māmā *i* te keke
The mother is baking the cake
Kei te kai ahau *i* te riwai.
I am eating the potato
In a passive sentence we mark the person doing the action, which we call the agent, with *e*.
Kei te pānuitia te pukapuka *e* Rangi.
The book is being read by Rangi
Kei te tunua te keke *e* te māmā
The cake is being baked by the mother
Kei te kāinga te rīwai e au.
However, in an active sentence you don't need to have an object.
We can just say
Rangi is reading, the mother is baking and I am eating, without being to say what Rangi is reading - could be a newspaper or a magazine - or what the mother is baking - could be biscuits - or what i am eating - could be an apple.
So we can just say
Kei te pānui a Rangi
Rangi is reading
Kei te tunu te māmā
The mother is baking
Kei te kai ahau
I am eating
In the same way, in a passive sentence, the agent, or the one doing the action, is optional.
We can say
The book is being read, without saying who is reading it
The cake is being baked, without saying who is doing the baking
And the potato is being eaten, without saying who is eating it.
So...
Kei te pānuitia te pukapuka
The book is being read
Kei te tunua te keke
The cake is being baked
Kei te kāinga te rīwai the potato is being eaten
And this is one of the main differences between active and passive sentences.
In an active sentence we have to say who is doing the action. Rangi, the mother and me. And we don't have to say what they are doing it to.
In a passive sentence it's the opposite. We have to identify the thing that is having something done to it - the book, the cake, the potato - and we don't have to say who is doing it.
And that's because the focus is an active sentence is the person - *the man* is sweeping - and in a passive sentence the emphasis is on the thing - *the floor* is being swept.