English employs three verb moods–indicative, imperative and subjunctive.
Indicative mood is for simple statements, while the imperative is for commands: “Run!”
However, it’s the subjunctive, represeting a wish or untrue situation, that befuddles virtually every English speaker.
Remember the line from the song in Fiddler on the Roof, “If I was a rich man…”?
Completely wrong verb usage!
Since the singer (“I”) is expressing an untrue situation, or a wish, the verb must be changed to the plural subjunctive form, were: “If I were a rich man….”
If can be a big indicator that the subjunctive mood is called for, but not invariably. I wish is a definite call for the subjective: “I wish you were more serious.”
For more examples, peruse this handy guide.