Do not cry ____ spilt milk.Fill the space with the appropriate preposition.
The only preposition that actually goes properly with this is "over." This is not the only preposition that would be grammatically correct, but it is the only one that properly completes the saying. "Do not cry over spilt milk" is a set saying.
If you were just trying to be grammatically correct, you could say "Do not cry about spilt milk." This would be correct in terms of grammar but it is not an actual proverb the way the other one is. I suppose you could even say "Do not cry on spilt milk" but that would sound pretty strange -- as if your tears were going to land on the milk.
Prepositions in English grammar are a class of words that reveal relationships between nouns, and pronouns. Most often they come before a noun. Their form remains unchanged, regardless of the case, gender etc. of the word they are referring to.
I think that you have the idiomatic expression "do not cry over spilt milk" in mind. In which case the only suitable preposition that would be right is 'OVER.'
The idiomatic expression which has become a cliche by being used countless times means that once some one spills milk on the floor it would be impossible to again collect all the milk which has been spilt. The only alternative is to forget about the spilt milk. So also in life it is not worth worrying over something which has happened and which cannot be set right.