_As--as_ for _So--as_. "He is not as good as she." Say, not so good.
In affirmative sentences the rule is different: He is as good as she.

_As for_ for _As to_. "As for me, I am well." Say, as to me.

_At Auction_ for _by Auction_. "The goods were sold at auction."

_At_ for _By_. "She was shocked at his conduct." This very common
solecism is without excuse.

_Attain_ for _Accomplish_. "By diligence we attain our purpose." A
purpose is accomplished; success is attained.

_Authoress_. A needless word--as needless as "poetess."

_Avocation_ for _Vocation_. A vocation is, literally, a calling; that
is, a trade or profession. An avocation is something that calls one
away from it. If I say that farming is some one's avocation I mean
that he practises it, not regularly, but at odd times.

_Avoid_ for _Avert_. "By displaying a light the skipper avoided a
collision." To avoid is to shun; the skipper could have avoided a
collision only by getting out of the way.

_Avoirdupois_ for _Weight_. Mere slang.

_Back of_ for _Behind_, _At the Back of_. "Back of law is force."

_Backwards_ for _Backward_.