In a word, yes, you can swap dried yeast for fresh. The main difference between fresh and dried yeast is stability, although purists swear by fresh and insist it's superior in both flavour and leavening power. Fresh yeast is available from health food shops and some delicatessens in compressed form. The fresher it is, the better, as it becomes less potent with age. It should smell sweet and have a soft texture. If it's crumbly or grey, bin it. It's best stored in a small screw-top jar in the fridge for no more than two weeks. If you find your fresh yeast is not up to scratch, don't despair. You can substitute dry yeast, a most useful store-cupboard staple (although it, too, is best kept refrigerated). If a recipe calls for fresh yeast and you only have dried, use half the quantity specified for the fresh. If you have dried yeast granules, you'll need to dissolve them in a little lukewarm water. If you have instant dried yeast, add it directly to dry ingredients.