Drawing trumps 

There are two reasons you may not want to draw trumps right away. It may be because you need to either ruff (trump) a loser/s in the dummy, or you may need to discard a card or establish a suit which then gives you a discard. The primary and most important objective for playing in a trump contract is to maintain control during the play. Without a trump suit the opposition may be able to win many more tricks in a suit in which they have a strong holding whereas if you are in a trump contract if you run out of their suit you can trump their winners.

The rules about drawing trumps are

1. Draw trumps straight away especially if you have concerns that your opposition may ruff (=trump) winning cards you hold in your side-suits.
2. Be careful not to draw trumps prematurely that is before you have ruffed cards in dummy that otherwise will be losing cards.
3. If your opposition have just an outstanding top trump card it is not necessary to draw it, you are much more concerned to draw out their losing trump cards before they can use them against your side suits.