Most common errors that I see students struggle with right through to Advanced level are:
1. Articles (very confusing for anyone who doesn't use articles in their own language)
2. Countable & Uncountable language - eg many, much, lots of, any
3. Third Person verbs - I go, she goes - it has, I have
In the U.S., American English speakers routinely confuse the two verbs to lie and to lay.
Then you get people who think they sound educated if they use the whom form. The problem is, they do not understand the difference between who and whom, so they end up saying whom when it should be a who. Few things sound as bad as saying whom when the correct form is who. You are better off always using who if you do not understand the rules for who and whom.
We also have native English speakers who have grown up in homes where nobody ever uses the participle form of the verb when using the present perfect tense.
They will say:
Have you ate yet? (Have you eaten yet?)
Have you ever went there? (Have you ever gone there?)
And let's not forget the people who write "I would of really liked that" instead of "I would've really liked that."
Here's a funny story I read in a book a few years ago:
There was this guy who was appearing in court because he was involved in some sort of disturbance.
"So you were shot in the fracas?" he was asked.
"No, your Honor," he replied. "I was shot right above the fracas."
when my classmates and i talked in english class it was a usual mistake that we told "i am agree or they were agree on". we consider agree as a noun because in farsi we use agree with another verb like tobe verb.
and i always forget "s" for verbs in simple present tense for he, she and it.
''None are enough for me'' instead of ''None is enough for me'' in answer to the question ''How many do you want?'' [This is so common a mistake that perhaps it has now become good English.]
Likewise with 'refute' which is used in the (wrong) sense of 'deny'; ''He refuted the allegations made against his party'' (=He denied the allegations made against them). Correct usage: ''He refuted Goldbach's conjecture''. (He proved that Goldbach's conjecture was false). Regrettably one needs to say ''He confuted Goldbach's conjecture'' as ''refute'' is now no longer available. Again, [ ] applies to this, too.
'More better' instead of 'better' or 'more good'. 'More heavier' instead of 'heavier' or 'more heavy', etc. This sounds bad to me. [ ] is coming to apply to it, too.
'may' instead of 'can' as in 1'May I help you?' instead of 2'Can I help you?'. 1 is, presumably, a misdirected super polite way to say 'can' here. 1 and 2 both mean, I guess, 3'Am I able to help you, and if so, may I?' Clearly, you do not want to say 3 every time you answer the phone, but you can use 1 or 2. 2 is better, since if you cannot help, then 'may' is out of the question.
If a native English speaker makes any of these mistakes I say nothing. They are usually not a learner and it is too bad for them.