Here are some of the most common mistakes native speakers of English make every day:
- Confusing the three principal parts of irregular verbs--"drink, drank, drunk," for example, or "lie, lay, lain." Non-native speakers learn right off the bat the second part is for the past simple and that the third (participle) is for perfect tenses, passive voice and other grammar constructions. "I have drank," is incorrect but quite common.
- Confusing elements that are phonetically similar: e.g. "could've" sounds exactly the same as "could of" does. A non-native speaker probably wouldn't do this because it just doesn't make any sense grammar-wise, and they pay more attention to each word that they say than your average native speaker.
- Double negatives--"I wouldn't know nothing about that"--but these tend to be a lot less frequent among educated native speakers and often associated with poor education. (You also find these among non-natives speakers, especially if double negatives exist in their own language!)
- Lack of awareness of countable and non-countable nouns: e.g. "There were less people there" instead of "There were fewer people there." However, a native speaker never refers to "informations" or "advices" as non-native speakers frequently do.
- Adverb/adjective confusion: "I feel horribly that.", "He drives very slow." (correct: I feel bad about that. He drives very slowly). Remember - adverbs describe verbs, and adjectives - nouns (there are, of course, some exceptions!).