After a lifetime of teaching English, I have found that “agreement” is the most perplexing part of learning any language – agreement of number, for example, between subject and predicate (“The people are/is shouting”) especially with irregular verbs or collective nouns, and agreement of gender in pronouns (Maria held her/his hand”), etc. I think this is because students think that knowing the “vocabulary meaning” of a word is enough. (This is a problem in all languages. For example, in French, “le critique” and “la critique” seem to mean the same thing, but they do not.) Following close behind are homonyms and syntax errors; homonyms such as “its” and “it’s” are particularly perplexing.