As an English teacher I have seen it all, it seems. Over the course of nearly 15 years of teaching, I have seen many mistakes and have corrected my fair share of lazy usage. Unfortunately students often write like they speak or use texting language on everyday writing assignments.
The following are some of the most glaring errors:
1. Could of - This usage error comes from bad usage in speech flowing into the written word. What students are saying when they say "could of" or "should of" or "would of" are the contractions could've, should've and would've. These contractions are lazy in themselves because they really mean could have/should have/would have. Using contractions is usually not allowed in formal writing anyway, so using could have/should have/would have is more desirable and correct.
2. You/Your - I absolutely hate person shift. It is probably one of the biggest problems in usage found in student writing today. Why do students use it? We could fold in using "like" as a filler as well. I point to the mighty and incredibly relevant Taylor Mali for this one. Watch the video and then continue with my commentary afterward.
Use of second person assumes that the reader automatically agrees with the writer and therefore rudely forces the reader to follow along. It is not used in academic writing at all.
3. Wanna/Gonna - Please spell out words rather than shortening them into strange Frankenstein hybrids of their former selves. "Wanting to" and "going to" are also misused. I would write "I am going to the store." I would not write "I am going to get my baseball glove". Possibly if a person were to say this, they do not need to tell us that they are "going" or "moving in a general direction toward" getting their baseball glove. "I am grabbing my baseball glove" is much better because we have used a more descriptive verb than "getting". "Wanting" is like "wishing" and is not specific enough for a formal paper.
4. There/Their/They're - Learn the difference between these three words because using them properly makes all the difference. "There" means direction as in "Put the plant down over there." "Their" means possession as in "Their car is a 1975 Pinto." "They're" is a contraction meaning "They are" as in "They are not able to tell the difference between these three words."
5. Loose/Lose - To "loose" is to "release from a grasp" or the opposite of "to tighten" or to "release" something. To "lose" is to not win or to misplace something.
These are my five. If you would like to list any other usage faux pas not discussed in this post, please do. We can all learn to work harder at this craft of writing.