Here are some mnemonics:
- They’re is short for They are. An apostrophe is ALWAYS a contraction*. It is a replacement for one (or sometimes more) letters, e.g. I’ll is short for I will, and the apostrophe takes the place of wi. In the case of They’re, the apostrophe takes the place of the a in are.
- Their is about possession. “Whose book is that? It’s theirs.” The way to remember this is that Their contains both a he and an I, which are pronouns for people. Their is about people’s things.
- There is about location. The way to remember this is that There contains here, which is a location. “Is it here? No it is there.”
*For people who get confused by an apostrophe indicating possession, e.g. "it is the king's crown".An old outdated version of English used to use -es to indicate possession. So the crown of the king was the kinges crown. Somewhere along the way, the e in kinges was contracted with an apostrophe. This fascinating blog from Merriam Webster describes this in more detail.
Mnemonics and Grammar Corrections
It is not the worst thing in the world to misspell words. It’s okay if you mess up they’re, their, or there. Maybe English is your second language, or maybe you just made a mistake.
Take it from someone who writes for his living — people who correct other people’s grammar for a sense of superiority are losers. As long as you are able to convey the message you’re trying to describe, you are fine.
However, if you ever find yourself confused, take a second to use mnemonics (memory devices and tricks) like the ones above. They may sound silly, but that’s precisely why they work — they’re memorable. Mnemonics enter our subconscious quickly, and after one or two uses, you’ll never be confused about the right word again.