Small slips can undermine a good piece of writing. These pairs are worth learning.
Affect and effect
Affect is usually a verb meaning to influence: “The weather affected our plans.” Effect is usually a noun meaning a result: “The effect was dramatic.”
Its and it’s
Its shows possession: “The company changed its policy.” It’s is short for “it is” or “it has.” If you can replace it with “it is”, use the apostrophe.
Then and than
Then relates to time: “First we ate, then we left.” Than is used in comparisons: “She is taller than him.”
Their, there and they’re
Their shows ownership, there refers to a place or is used in “there is”, and they’re means “they are.”
Fewer and less
Use fewer with things you can count and less with quantities you cannot: “fewer cars, less traffic.”
A tip
When unsure, say the sentence in full and replace the word with its meaning. If it still makes sense, you probably have the right one.
