Hi,

Mai ko kela khana tha.
Mai se kela khaya nahi gaya tha.
Mai ka ek kela hai.
Mai ne pani piya tha.

I understand the direct English translations of these sentences. What
I don't know are the functions of "ko", "se", "ka" and "ne", and to
what word in the sentence that each of them is "attached" to
(grammatical cases). In particular, I need good explanations about
when to use "ko", "se", "ka" and "ne".

I would be very happy if you know Hindi. Others helpful comments are welcomed too.