Ruffing when your partner is master
Usually no: if your partner already wins the trick, you don't have to ruff it. But this is a convention, not a universal law.
The principle
When the trick already belongs to your team because your partner is master (they played the current highest card), ruffing to win an already-won trick is pointless. The most common convention therefore lets you skip the ruff and discard a useless card instead.
Why it helps
Not wasting a trump on an already-won trick means keeping your trumps for the tricks that matter. You can use the chance to discard a weak suit, or instead drop a big card (10, Ace) on your partner's trick to load it and score more points.
Careful: it's conventional
Not every table allows this exemption. In the strict "must ruff" version, you must ruff whenever you can't follow suit, even on your partner's trick. Before a serious game or tournament, confirm the rule in force to avoid a renonce claim.
See also
FAQ
Must you ruff when your partner already wins?
Usually no: the common convention lets you skip ruffing your partner's trick and discard instead. But some tables still require you to ruff.
What do you play if you don't ruff your partner's trick?
You can discard a weak card, or load the trick by dropping a big card (10 or Ace) to add points to your team's total.