Re: Sentence error
Do you speak any foreign languages? If so, think about the kinds of errors you make with that language and consider if they're the same kind of errors learners make in English.
Do you already know English learners and talk to them regularly? That will give you a great idea of some of the errors they make and many of them are quite good at explaining why they chose the word or the construction they did.
Remember - correct capitalisation, punctuation and spacing make posts much easier to read.