Confusing Word Pairs

Here are some tips or reminders about certain confusing words in English.

Its or it's?

It's is a contraction for "it is" or "is has."
It's understandable she went away.
It's been a long time since we've seen you.

Its is the possessive form, and yes, it lacks an apostrophe.
The butterfly was drying its wings in the sunshine.

Ones or One's?

Ones refers to specific things or to numbers.
No, I don't want those; I want the ones over there.
On her eleventh birthday, there were two candles on the cake shaped like ones.

One's can be a contraction, as in One's never sure which is better, or a possessive: One can't have one's cake and eat it, too. This is confusing because it doesn't follow the same pattern as its and it's. So it's both easier to remember, and more correct, to use "his" as the possessive of the pronoun "one." One can't have his cake and eat it, too. That way, you can remember, no apostrophes with any possessive pronoun: mine, my, your, yours, his, hers, its, ones, ours, theirs. No apostrophes. Except with proper nouns: Sam's, Brittany's, Doug's.

Lay or lie?

Lay is a transitive verb, meaning it's something you do to an object.
Please lay the forks on the lefthand side of the plates.
He laid his cards on the table. (Past tense)
I have never laid eyes on him before. (Past perfect tense)
Now I lay me down to sleep. "Me" is the object upon which the verb is acting.
His hens suddenly stopped laying. Here, the object, eggs, is implied.

If you mean to recline, the word in the present tense is lie.
Lie down now and take your nap.
Let's lie out in the sun a little while.
Do you sleep lying on your back or stomach or side?
When I was in Spain, I lay down for a siesta every afternoon. (Past tense, and where the confusion comes in!)
I've never lain in a more comfortable bed. (Past perfect tense)

Can you think of other confusing word pairs we all really ought to learn once and for all?