Translate a Confusing Grammar Rule Into One Plain English Sentence

Textbook grammar explanations often use more jargon than the rule itself needs. This takes the confusing version and the example that trips you up, then gives back one plain sentence stating the rule properly, with two short examples showing it in use.

You are stuck on a grammar rule that every textbook explains in the same confusing way. Paste [the rule and an example sentence that trips you up], and get back one plain English sentence that states the rule the way a fluent speaker would actually explain it to a friend, followed by two short example sentences that show it working.