Breathtaking can also be spelled breath-taking.
No, but it can be spelled "hard tack" with a space.
close by or close-by
No.
My educated guess is that it is used without a hyphen when used as a verb, and with a hyphen when used as an adjective.Dr. Smith is on call. He is the on-call physician.Similar to follow up:Dr. Smith...
How to Use Hyphens and Dashes
Adobe Creative Suite 2 - How to Adjust Leading, Kerning, Spacing and Alignment