As you want me to be a bit more specific, I am happy to talk about some areas of grammar that often cause problems for students.
The Present Perfect is an area of grammar that causes problems for students at all levels. A tense that involves a connection between the past and the present can be very confusing for students (especially when a lot of languages either don't have an equivalent tense, or they have a tense that appears similar in form but is used quite differently).
Articles are, of course, a problem at all levels as well. Students of all nationalities make mistakes with articles but they tend to make different kinds of mistakes (depending on their native language). Some nationalities tend to use them too little when they speak English (for example, Russian and Japanese speakers tend to leave them out) whereas other nationalities tend to use them too much when they speak English (for example, Spanish/Italian speakers). It is, obviously, understandable that they make these specific types of mistakes due to features of their native languages.
Gerunds and infinitives is another grammar point that causes problems for a lot of students. Unfortunately, there are some verbs that take a gerund (eg 'enjoy + gerund') and others that take an infinitive (eg 'need + infinitive'). Unfortunately, there is no clear reason for this and students need to learn them through exposure and practice.
These are just a few examples of difficulties that students have.
Needless to say there are many other aspects of English that cause problems for both students and teachers, and I'd be interested to hear other people's opinions.