What are the most complicated parts in learning English grammar to students?

The hardest parts to learning any foreign language that is not your own are features of the language which are not predictable from experience. Many languages have immensely complex systems of morphology that simply must be learnt and cannot be predicted based on your knowledge of the world or first principles.

While English largely lacks such complex morphology, it makes up for it in complex syntactic structures and idiomatic constructions that are rare or absent in many other languages.

Teaching grammar can be difficult in terms of sentence structure.Rules can be easier for learners to frappa.

Speaking English is the hardest, grammar comes later.

Active and passive voice is most difficult part in English grammar......

The difficulty or simplicity for a particular aspect of language learning will depend on the context.  Some learners may handle drills more easily than structure.  For example, when moving from drills to sentences, paragraphs and essays, language teaching becomes a little more complex. In other words those drills cannot be translated into writing short sentences and essays.  Progress with grammar comes with reading and understanding the language.  A lot of effort should be placed into in the initial preparation.  Grammar becomes easier once the language is known or there is a more developed understanding of the language.  Second language speakers will always struggle with grammar. 

The correct pronunciation is the biggest problems, also understand the words

Teaching grammar can be difficult in terms of sentence structure; correct written expression ...

Phrasal verbs are the most difficult to teach to students. Since there are not only a vast number of them and they also have multiple meanings, students just have to learn them by their context and memorize them the best they can.

I suppose in any foreign language learning experience all the aspects of the target language are equally difficult or equally simple, though there may be some difference in degrees of difficulty depending upon what interests the learner the most. For instance, some people may find accent and intonation very tough, to others it  may be very pleasing. So, there cannot be any generalizations; but still people in general find grammar of a new language difficult because commonly it is taught out of context which doesn't make sense to many people. Why do people find it easy to learn a foreign language in the native setting? Because there they "imbibe" the grammar of the target language, not learn it through consciously efforts.