Grammatically, the subjunctive is a verb mood, not a verb tense. Most sentences use the indicative mood; the subjunctive in English has fairly restricted uses. Often, subjunctive forms don't look any different and mostly you'll know which form to use because it "sounds right." But there are a few places where people run into problems.
Here's when to use the subjunctive:
1. In subordinate clauses for demands, suggestions and necessities. These are generally straightforward.· The teacher suggested that Johnny pay more attention to his use of apostrophes.· Oda Mae Brown asked that no one speak during the seance.· It is crucial that everyone refrain from getting water near the witch.
2. In subordinate clauses for expressions of wishing, hoping, etc., where what you are wishing for probably isn't so. This is the "Wish you were here" construction.
· We wish they wouldn't chew with their mouths open.· She hoped that her brother knew better than to lick a frozen flagpole.
3. In "Let …" constructions, also called the "hortatory subjunctive." This is the "Let them eat cake" construction.· Bubba fixes birdhouses for a living; let him fix yours.
4. In sentences when you are talking about something contrary to fact. This is the "If I were you" construction, and is where things can get tricky.
Past tense Use the "had ___" form of the main verb, and the "would have ___" form of the secondary verb.· If Cedric had bought a Porsche (but he didn't), we would have known he was having a midlife crisis.
Present tense Use "were" or the "were ___-ing" form of the main verb, and "would ___" for the secondary verb.· If Mindy were working tonight (but she's not), she would be mad about the cookie crumbs we left all over her desk.· Suppose Mindy were here (but she's not) — what would she say about the mess we've made?
Future tense This is where things get tricky, because you want to distinguish between things that might happen and things that won't.· If Sylvester catches the mouse (possible), he will get a treat.· If Sylvester were to catch the mouse (but he won't because he sleeps all day), he would get a treat.
Be careful to distinguish a subjunctive from a conditional.· The bill would make it illegal to smoke indoors. (it hasn't passed yet)· If the bill becomes law (it might), smoking will be banned indoors.· If the bill were to become law (it won't, because the governor has promised to veto it), indoor smoking would be banned.