This PowerPoint presentation covers everything your students need to know about using the subjunctive with expressions of doubt and adjective clauses.
(accompanying notes sheet to give your students is available here
It begins by recapping the subjunctive in general, how it is formed, and which verbs are irregular. (So, ideally your students have already been introduced to the concept of subjunctive when you show them this. Maybe they just haven't seen it used w/ expressions of doubt before.)
First, they are asked to do some practice conjugations (all subjunctive), just to make sure they are forming the verbs correctly. There are ten practices sentences. Then, there is another set of ten that combines subjunctive with regular/indicative, so that they become adept at recognizing the difference.
There is also a short section on adjective clauses at the end; for example, why do we use subjunctive with sentences like "I'm looking for a house that has four bedrooms" ? Because until you find such a house, it is hypothetical!
I had great success using this PowerPoint with my students and think that they really got the picture by the end of it. Takes about an hour (one standard class period) to work through it. I hope it works for you!