It’s one of the most common mistakes in the English language, and it drives a lot of people up the wall. If you’ve ever read a rant by someone on Facebook or Twitter, chances are you’ve seen them misuse their/they’re/there. But how did it end up that way?
It all started with Old English. The word for “there” was spelled þǽr (thǽr). The Old English word for “their” was hiera, so no one was having any trouble telling those two apart. When Scandinavians started coming to the British Isles around the year 1000, the locals started incorporating their words into English. One example is their word for “their”, þaire (thaire). Now there were two words that were similar, but had different spellings and pronunciations.
Over the next several centuries, the English language was standardized somewhat through more development and the invention of the printing press, which led to higher literacy levels.
“There” changed spellings many times, including thar, thaire, ther, yar, theer, thiar, and thore. “Their” was alternately spelled as thayir, thayre, yaire, and theer. Over hundreds of years of changed spellings, we ended up with two words spelled differently with the same pronunciation. And then there was “they’re” (confused yet?). Contractions weren’t written like this until the late 16th century, and “they’re” naturally became the short spelling for “they are.” So English speakers ended up with three words that all sound the same but have different meanings.
Thanks a lot, ancestors.