What must you do with frozen phyllo dough before using it?

Thawing frozen phyllo dough completely is essential before use because it ensures that the layers of dough become pliable and easier to handle. When phyllo dough is frozen, it becomes rigid and can easily tear or break when you attempt to work with it without adequate thawing.

Once completely thawed, the phyllo dough can be carefully separated and layered, allowing for the desired flaky texture in pastries. This is particularly important because phyllo relies on its delicate layers to achieve the light, crisp finish that characterizes many recipes, such as baklava or spanakopita.

The other options provided are not suitable methods for preparing phyllo dough for use. Microwaving can lead to uneven thawing, potentially causing portions of the dough to become too warm and sticky, which makes it difficult to work with. Cooking phyllo before using would defeat its purpose, as it's meant to be baked after assembly to achieve its signature crispness. Soaking it in water is also inappropriate because it would cause the dough to become soggy and lose its flakiness, which is crucial for a successful pastry.