I wish it rained is now in the present. I wish it would rain is in the future. Maybe you would like to take a look at this page: http://www.englishexercises.org/makeagame/viewgame.asp?id=5841 Keep up your good work in learning English!

I would say I wish it had rained - past tense I wish it would rain - present and future

'I wish it rained' is not correct grammar.