1. affect / effect
Today’s storm will affect air traffic, but it should not have an effect on tomorrow’s flights.
affect (verb: to change or influence something), then effect (noun: result of an action).
2. eager / anxious
I’m anxious about my performance review, but I’m eager to have it behind me.
anxious (feeling worry or unease), then eager (keen interest or desire)
3. ensure / insure
To ensure that you can call a doctor when your children are ill, we will insure your entire family.
ensure (to make certain), then insure (to safeguard against loss, usually by securing insurance coverage)
4. farther / further
The farther they walked, the further engrossed they got in conversation.
farther (greater distance), then further (to a greater extent)
5. fewer / less
Fewer people signed up for the event than we had expected, so we’ll need less food.
fewer (things you can count), then less (quantity uncountable in terms of number)
6. its / it’s
It’s up to you, but I hope you’ll leave the key in its usual place.
it’s (contraction for it is), its (possessive of it)