There is one particular grammar mistake that really irks me: a/an usage.
People that misuse them tend to fall into 2 groups:
Native English speakers that tend to overuse "a".
- Examples: a apple, a egg, a ice cream cone, a octopus, a umbrella
Non-native English speakers that tend to overuse "an".
- Examples: an ball, an computer, an shirt.
The native English speakers bother me more than the non-native ones, since this is something you should have learned early on in grammar school.
The other one that 'irks' me even more is Me versus I
All through grade school the teachers were pounding into the students, "Mary and I go to the market".
Yet, today, I cringe when I hear a statement like, "At the party it will only be William and I"
Subject versus* Predicate.
*Writing this post reminded me of two more (seen in written language):
per say (to mean per se)
verses (to represent versus)
And then there is the ubiquitous, "ect." Which shouldn't be a problem for anyone who has worked on the command line in Unix