What is the most complicated and most difficult grammar rule for your students?

There are many grammatical areas that are very challenging to my students : mixed conditionals and the subjunctive form

Thanks for the invite,

Most of the students get confused regarding the difference between the past and the present perfect. They do not believe that the present perfect remains, in a way or another, present!

It varies from student to student but generally tenses are the one that keeps them tense. Grammar is a facet of English which needs revision from time to time until you become fluent.

Using the data from TrackTest.eu English Online Assessment Centre, questions about idioms and phrasal verbs cause the most problems.

When you ask specifically about the grammar rule, correct use of conditionals/If clauses is the most difficult for our users based on our stats. Please be aware that it might depend what is your mother tongue.  

It is relative, for some students it is difficult to handle concord, agreement of verbs with the subjects, for some others "sub-ordinate and co-ordinate conjunctions"