if you want to ask someone if they would like a cup of tea, you could ask: He kaputi koe? And they could answer; Āe, he kaput ahau.

But, you would be asking: "Are you a cup of tea?" and they would be replying: "Yes, I am a cup of tea!"

So, you need to ask:

He kaputi māu?
Would you like a cup of tea?

And they could answer:

Āe, he kaputi māku.
Yes, I would like a cup of tea.

And this introduces us to the idea of unrealised possession.

Imagine you but something on-line, from Amazon, say. You have ordered it, and paid for it, but it is still in transit. You own it, but you don't yet possess it. It's yours, but you don't yet have it in your hands. This is unrealised possession. And, in Māori, we would use a word like māku to describe it: a parcel from Amazon māku. It's mine, but I don't have it yet.

in English, we might say: "I will have a cup of tea". You could take this to mean: "in the future, a cup of tea will be mine, but, at the moment, I don't have it. The cup of tea belongs to me, but I don't have it yet." He kaputi māku.

We often translate a sentence with mā with the word "for":

Mā Mere tēnei pukapuka.
This book is for Mary.

Again, this means that she has future ownership of the book, but she hasn't got it yet.

Mā Rangi ēnei kākahu.
These clothes are for Rangi.

He will get them - he hasn't got them yet.

There are a family of words that start with mā or mō.

If something is for me, we use māku.

He kaputī māku.
I will have a cup of tea.

If we are talking to someone, we use māu:

He kaputī māu?
Will you have a cup of tea?

And if we are talking about a person, we use māna:

Māna tēnei pukapuka.
This book is for him or her.

Note that you do not need the i in these sentences to identify the object of the sentence,

For example, we say:

Māku te pukapuka.
The book is for me.

We don't say "māki i te pukapuka".

So, if I was saying that the children are going to the playgroud, I would use ki and say:

Kei te haere ngā tamariki ki te papa tākaro.
The children are going to the playground.

But if I was saying that the playground is for the children, I would just say:

Mā ngā tamariki te papa tākaro.
The playground is for the children.

I wouldn't say "mā ngā tamariki i te papa tākaro."

However, we also have the a and o categores for this family of words.

So if we are talking about something in the a category we would use:

māku
māu
māna

But if we are talking about something in the o category we would use:

mōku
mōu
mōna

So if we wanted to say this food is for me, because food is in the a category I would say:

Māku tēnei kai.
This food is for me.

But if I was talking about water, which is in the o category, I would say:

Mōku tēnei wai.
This water is for me.