When should I lead a trump?
Whether you should lead a trump depends on the sort of defense you'll want to execute on the hand. Broadly speaking, there are five lines of defense:


A forcing defense (you force declarer to ruff, making them run out of trumps while the defense still has them). This is a reasonable approach when you or partner hold four trumps, declarer is in a 4-3 fit, or declarer is known to have shortness. Leading a trump here is usually fatal.
A ruffing defense (you want to ruff declarer's winners in a side suit). This is a reasonable approach when you or partner are short in a side suit held by opponents. Usually you'll lead a singleton, or an ace in a suit partner holds a singleton in. Leading a trump is usually fatal.
An active defense (you try to take your side-suit winners before declarer can establish a side suit). Against a trump contract, you may take this approach when dummy has shown a long, strong suit, when opponents have shown extras and tried for (or bid) slam, or when you can see that declarer's second suit is breaking. Leading a trump is usually fatal.
A passive defense (you try to make leads that don't give away tricks). Against a trump contract, you may take this approach when you know or suspect that your tricks are safe, as long as you don't give declarer extra opportunities to finesse, create entries in dummy, etc. Leading a trump here is often safe.
Cutting down ruffs (you try to prevent declarer from ruffing tricks in a side suit). This is often right when the auction indicates opponents are in a misfit, or when you have good cards in trumps, or when dummy has shown shortness in a side suit. Leading a trump here is often required.