1) ALTOGETHER/ALL TOGETHER
“Altogether” is an adverb meaning “completely,” “entirely.” For example: “When he first saw the examination questions, he was altogether baffled.” “All together,” in contrast, is a phrase meaning “in a group.” For example: “The wedding guests were gathered all together in the garden.” Undressed people are said in informal speech to be “in the altogether” (perhaps a shortening of the phrase “altogether naked” ).
2) ALUMNUS/ALUMNI
We used to have “alumnus” (male singular), “alumni” (male plural), “alumna” (female singular) and “alumnae” (female plural); but the latter two are now popular only among older female graduates, with the first two terms becoming unisex. However, it is still important to distinguish between one alumnus and a stadium full of alumni. Never say, “I am an alumni” if you don’t want to cast discredit on your school. Many avoid the whole problem by resorting to the informal abbreviation “alum.”
3) AMATURE / AMATEUR
Most of the words we’ve borrowed from the French that have retained their “-eur” endings are pretty sophisticated, like “restaurateur” (notice, no “N”) and “auteur” (in film criticism), but “amateur” attracts amateurish spelling.
4) AMBIGUOUS/AMBIVALENT
Even though the prefix “ambi-” means “both,” “ambiguous” has come to mean “unclear,” “undefined,” while “ambivalent” means “torn between two opposing feelings or views.” If your attitude cannot be defined into two polarized alternatives, then you’re ambiguous, not ambivalent.
5) AMBIVALENT/INDIFFERENT
If you feel pulled in two directions about some issue, you’re ambivalent about it; but if you have no particular feelings about it, you’re indifferent.