Is "substract" (versus "subtract") a proper word?
Solution 1:
"Subtract" is the word. Though the obsolete word "substract" did exist, any occurrence you see these days is most likely just a common mistake, formed by analogy either with "abstract" or with other languages whose corresponding words do have two ‘s’s.
Many recent dictionaries do not list "substract". Of "substract", the Century Dictionary (1891) said:
- An erroneous form of subtract, common in vulgar use.
Webster's Revised Unabridged Dictionary of 1913 called it "obsolete", while the even older version of 1828 said
Note.--Substract was formerly used in analogy with abstract. But in modern usage, it is written according to the Latin, subtract. See this word and its derivatives.
Additionally, I know people who find "substract" very annoying. :-)
Solution 2:
It seems that the usage of "substract" is linguistically incorrect. However, I disagree about explaining this usage as a "showing-off". It seems that other languages do contain the letter "s" as in "soustraction" in French. People with a multi-lingual background are more likely to make mistakes, and it is nice if we just point that out to them without prejudice.