If I were a rich man (but I am not) I would ....
The subjunctive is used with modal verbs (could, should etc). Some of the rules around it vary but the key one is that it takes the infinitive without 'to' in the present tense e.g You should be okay.
For a very detailed usage and exercises go here:
English Language 100 FAQ - only £1.99
Advanced Grammar in Use With CD ROM
Common Errors in English Usage 2nd Edition
English Grammar For Dummies