There are many confusing words in English, but here are the two most frequently confused sets:
| Its: possessive
It’s: contraction (it is)
| Their: possessive
They’re: contraction (they are)
Another common confusion: effect vs. affect. The challenge here is that both words have multiple meanings and can shift which part of speech they are. Most commonly, though,
effect is a noun (e.g. the effect of something) and
affect is a verb (e.g. to affect the outcome of something).
(Effect as a verb means to cause something to happen, to bring something about; affect as a noun is mostly limited to psychology and means emotion or desire, especially as it influences behavior.)
We all have different sorts of words that confuse us, even the best writers and spellers. Learn yours and keep a list on your computer or near your desk to remind yourself.
For some more information on frequently-confused words, see this guide from the chief of copyediting at one of the largest publishers in the world, Random House.