To italicize a word is using a double edged sword. Doing so makes the author look snobbish to some readers and like he is looking down on the reader, a big no-no. Not to italicize a foreign word is to destroy the choice to use the word.
If a word or phrase has become so widely used and understood that it has become part of the English language — such as the French "bon voyage" or the abbreviation for the Latin et cetera, "etc." — do not italicize it. Often this becomes a matter of private judgment and context. For instance, whether you italicize the Italian sotto voce or French soup du jour depends largely on your audience and your subject matter.
1. Emphasis. When you want a word or phrase to stand out. Ex. “I told you not to go.
2. Word reproducing sounds. Grrr! Bzzz! Include to exclamation point.
3. Foreign words not understood by the general public. Ex: Guten morgan! “Au revoir meant forever,” she thought. “Be-gen-ee-tah”, said the Apache warrior.
4. Names of vehicles. The Orient Express, the Titanic, Endeavor but not makes of vehicles like Mercedes Benz or Ford Mustang.
5. The first time you introduce a foreign phrase but not again if you use it often.
6. Titles if they can stand alone, yes to book title, a story, TV show. Not italicized are books in a religious connotation. The Bible is not italicized nor the titles of chapter within it but it are capitalized. Smaller texts such as short stories from an anthology, journal articles, and episodes of television shows are examples of titles that cannot.
7. Some scientific and technical writing does but check in-house criteria. This medication is to be administered to the patient BID.