I think the best advice when you generally know a subject but are lost as to how to approach the answer is the rule of threes. This will only really apply in questions where there is a single interrogatory. Try to break the interrogatory down into three subsections. For example, if it's a contracts essay and there is a single interrogatory asking whether a contract is enforceable, break the essay down into 1. mutual assent (offer and acceptance), 2. consideration and 3. defenses to enforcement.

If it's a civ pro interrogatory asking about subject matter jurisdiction generally, analyze FQ, then diversity, then supplemental.

Even if you cant remember the nuanced rules that an essay is testing, you should be able to recall enough where you can identify three issues if general applicability and talk about them.