Grown is about growing: it means fully developed, or the past participle of grow. Groan is a sound, the low noise you make when you are in pain or fed up. If you can hear it, it is groan; if it has got bigger, it is grown.
The core difference
Grown describes something that has grown, while groan is a low, unhappy sound or the act of making one.
- grown — the past participle of grow, or an adjective meaning adult: She has three grown children living abroad.
- groan — a low, drawn-out sound of pain or annoyance, or to make it: She groaned when she saw the size of the pile of homework.
How to tell them apart
A simple trick: groan has an a in the middle, like moan, and both are noises. Grown has row in it, and things grow in rows, such as crops in a garden.
The mix-up shows up in writing because the two words sound the same when spoken. Look at the job the word is doing: if it describes size, age or a plant that has developed, you want grown. If it describes a sound, or someone reacting with displeasure, you want groan.