What can you use yeast for?

It depends. You should always proof the yeast by adding a little (about 1/4 tsp) to 1/4 cup lukewarm water. Add about 1/4 teaspoon of sugar and let it sit for 5-10 minutes. If the yeast is still alive (and suitable to use in baking) the water's surface wlll be bubbly which will indicate that the yeast will work.