Affect vs. Effect, Fewer vs. Less and Other Pairs That Trip People Up

Affect and effect

In everyday writing, affect is a verb meaning to influence, and effect is a noun meaning a result.

The weather will affect our plans. The weather had a big effect on our plans.

Effect can also be a verb meaning to bring about, as in 'to effect change', but you will rarely need that.

Fewer and less

Use fewer with things you can count one by one. Use less with amounts you cannot count.

fewer mistakes, fewer people, fewer emails less time, less water, less noise

Then and than

Then is about time or order. Than is for comparing.

We ate, and then we left. She runs faster than I do.

Lose and loose

Lose is a verb that means to misplace something or to fail to win. Loose is an adjective that means not tight.

Don't lose your keys. This screw is loose.

Advice and advise

Advice is the noun and advise is the verb. Notice the sound: advice ends in a soft s, advise in a z sound.

Thanks for the advice. I advise you to wait.

Principal and principle

A principal is the head of a school or the main thing. A principle is a rule or belief.

The principal welcomed the new students. Honesty is a principle I live by.

Every day and everyday

Every day means each day. Everyday is one word when it describes something ordinary.

I walk to work every day. These are my everyday shoes.

Could have, not could of

'Could of' is a spelling of how 'could've' sounds. The written form is always 'could have', and the same goes for would have and should have.