1) AM/PM
“AM” stands for the Latin phrase Ante Meridiem —which means “before noon”—and “PM” stands for Post Meridiem : “after noon.” Although digital clocks routinely label noon “12:00 PM” you should avoid this expression not only because it is incorrect, but because many people will imagine you are talking about midnight instead. The same goes for “12:00 AM.” Just say or write “noon” or “midnight” when you mean those precise times.
It is now rare to see periods placed after these abbreviations: “A.M.” , but in formal writing it is still preferable to capitalize them, though the lower-case “am” and “pm” are now so popular they are not likely to get you into trouble.
Occasionally computer programs encourage you to write “AM” and “PM” without a space before them, but others will misread your data if you omit the space. The nonstandard habit of omitting the space is spreading rapidly, and should be avoided in formal writing.
2) ABJECT
“Abject” is always negative. You can’t experience “abject joy” unless you’re being deliberately paradoxical.
3) ABOUT
“This isn’t about you.” What a great rebuke! But conservatives sniff at this sort of abstract use of “about,” as in “I’m all about good taste” or “successful truffle-making is about temperature control” ; so it’s better to avoid it in very formal English.
4) ABSORBTION VS ABSORPTION
Although it’s “absorbed” and “absorbing,” the correct spelling of the noun is “absorption.”
5) ABSTRUSE/OBTUSE
Most people first encounter “obtuse” in geometry class, where it labels an angle of more than 90 degrees. Imagine what sort of blunt arrowhead that kind of angle would make and you will understand why it also has a figurative meaning of “dull, stupid.” But people often mix the word up with “abstruse,” which means “difficult to understand.”
When you mean to criticize something for being needlessly complex or baffling, the word you need is not “obtuse,” but “abstruse.”