I agree with Jana. I would add that not all if clauses are conditionals. The example you gave, "John, if you would turn on the light there, please.", is not a conditional sentence, rather a request.
To expand on the second conditional, Celce-Murcia and Larsen-Freeman explain a hypothetical (present and future) and counterfactual conditional (present) as it pertains to the second conditional (1983, p. 343-344). Alternate verb forms with modals can be used in the if clause of a hypothetical conditional.
Example: If my sister should happened to have money, she would buy a dress.
This cannot happen in the counterfactual conditional.
Example: *If I should be Superman, I would fly around the world.
I highly recommended The Grammar Book for additional reading on grammar in general.
Celce-Murcia, M. & Larsen-Freeman, D. (1983). The Grammar Book: An ESL/EFL Teacher's Course. Boston, Massachusetts: Heinle & Heinle Publishers.