1. Can you say MAY instead of CAN?
2. Are there any other synonymous idioms?
The other thing in favour of 'can' is that it emphasizes the idea of doing as much as you can..
Another idiom - similar in some ways (emphasizing the limits of what can be done) but not meaning the same - is 'You can take the <noun> out of <place> but you can't take <place> out of the <noun>' - which refers to the influence of the place you're born in on who you are: a non-PC example is 'You can take the girl out of Essex but you can't take Essex out of the girl'.
I prefer Stan Laurel's variation, "You can lead a horse to water but a pencil must be lead."
And I prefer Gertrude Stein's () 'You can lead a whore to culture but you can't make her think.'
"This idiom appeared in literature over the centuries in a variety of forms;for example, in the play Narcissus, which was published in 1602, of unknown authorship, subtitled as A Twelfe Night merriment, played by youths of the parish at the College of Saint John the Baptist in Oxford:Your parents have done what they coode,
They can but bringe horse to the water brinke,
But horse may choose whether that horse will drinke. "
As a EFL teacher,I can interpret this idiom by saying that we cannot force a child or put pressure on him to learn if he is not willing and ready to do it.
"People, like horses, will only do what they have a mind to do"