1. Confusion between countable/uncountable (e.g., saying the incorrect "less people" rather than the correct "fewer people"); 2. Incorrect use of "I," "me" & "myself" (e.g., mistakes like, "They gave a gift to my wife and I," or "John and myself went to the football game"); 3. Using "would have" twice in the third conditional, rather than "had/would have" (e.g., the incorrect, "If I would have known you were coming, I would have taken a shower"); 4. Subject/verb agreement (e.g., the incorrect, "Either tea or coffee are fine"). Sometimes this is done to avoid awkward constructions like "his/her" or "he/she" by using the plural "their" (e.g., the incorrect "Each student should put their name on the list"). As for grammar, once you reach secondary school you don't study it for speaking, only for diagramming sentences in an academic language course (because you've already learned grammar when you learned your native language). Besides, even if you learn a grammar rule like "don't end sentences with a preposition," it probably won't change your day-to-day speech.

I'd say things like "there / their / they're" or "your / you're"... and words like "necessarily". Also, many things are acceptable when talking casually, such as "wanna", which isn't actually a word.

This is worth looking at; if you make these mistakes, then feel proud you are making mistakes at an English-speaker level. :P http://theoatmeal.com/comics/misspelling As well as the usual misspellings and mistakes already mentioned, there are mistakes such as using a plural after singular "there's" or "where's" in speech (Where's my pencils? There's many people here.), and other phonetic mistakes, the most horrible being to write "should of" (or could/would of) instead of "should've". Add to that the generally horrible invasion of sms abbreviations when it's not appropriate, ie. in proper writing and even short comments. (I even teach lessons on how to decode it.)