Writing formally (for academic essays, business letters, resumes, etc.) doesn't mean trying to sound smarter or using bigger words--you don't want your voice to sound unnatural. Formal writing is more about avoiding certain things: cliches, colloquial expressions, contractions, and commonly misused words.
A cliché is a phrase, expression, or idea that has been overused to the point of losing its intended force or novelty, especially when at some time it was considered distinctively forceful or novel.
since the beginning of time
since the dawn of time
as luck would have it
beat around the bush
best foot forward
dead as a doornail
easier said than done
calm before the storm
slept like a log
wrong side of the bed
in a nutshell
we are talking like
broke (referring to money)
cool or hot (not referring to temperature)
put someone down
It’s (you can use the possessive pronoun its)
There vs their I am going there tomorrow / It is their car.
Our vs are They are similar to our family.
Affect vs effect verb: The writing affected me / You affect your writing / Life affects your writing.
noun: the Doppler Effect, the effects of the storm.
Were vs where They were going to the place where they go every year.
Than vs then You write better than me/First we dance and then we sing.
Click here to see more commonly misused words.