The BBC News online magazine carries the following user-contributed list of 20 common grammatical errors. At some points BBC either corrects or adds further clarification to the suggestions. I will list those suggestions in bold type so you can look them up at your leisure. Here they are:

    1. Confusing “have” and “of”, as in “I could of learnt how to write properly.”
    2. Using the phrase “for free” instead of “for nothing”.
    3. Writing “12pm” when “am” means “ante-meridiem” (before noon) and “pm” means “post-meridiem” (after noon). One should use either “midday” or “noon”.
    4. “Effect” versus “Affect”.
    5. Haphazard applications of apostrophes, as in “CD’s” (I’m assuming “CDs” would be the proper use).
    6. Using “I” where “me” is correct, as in “She said some very kind things about George and I.” (Crystal has corrected I — I mean me — a number of times on this one.)
    7. Incorrectly using reflexives, such as “yourself” or “myself” when “you” or “me” is correct. (I just did this recently. I said something like “Myself and another guy are being mentored by one of the pastors.” Ack! Away with the pomp and frillery!)
    8. Saying “none of them are” instead of “none of them is”.
    9. Avoiding varied prepositions such as “similar to“, “different from“, and “compared with” by slapping “to” on every phrase.
    10. Saying “Then they opened fire on us” when it should really be “Then they open-fired on us”.
    11. Using “literally” incorrectly, as in “I literally went blue with anger.” People don’t literally turn blue.
    12. Confusing “its” and “it’s”, the former being the possessive form, and the latter being the contraction of “it is”. (I still remember getting this wrong on a test I took when I was a young chap. It’s plagued me ever since.)
    13. Employing the phrase “due to” when “owing to” is meant.
    14. “They’re”, “their”, and “there”. Oh, and “to”, “two”, and “too”, too.
    15. Confusing “lend” and “borrow”, as in school children asking “to lend your pencil” when they really mean “to borrow your pencil”.
    16. Saying “amount of people” instead of “number of people”.
    17. Telling someone you went to a place “by foot” rather than “on foot”.
    18. Using a singular noun with a plural verb, as in “The team are happy with their victory.”
    19. Trying to avoid ending a sentence with a preposition when you’re using a verb that includes a preposition, such as “set up”. For example, trying to say “…I am using a new computer up with which my manager recently set me” instead of “At work I am using a new computer with which my manager recently set me up.”
    20. Saying “stadiums” rather than “stadia” as the plural of “stadium”.

One I would add is saying “The car needs washed” rather than “The car needs washing” or “The car needs to be washed.” I grew up thinking nothing of the first construction. When I came to college, however, a friend told me that was incorrect. I was aghast.

Are there any you would add?