What are the difficulties that usually face students when learning english?

The most common is the practice of spoken

one of the problems is the influence of the mother tongue on the target language (English)specially in the Arab communities some learners think in Arabic and try to translate it into English from here mistakes begin. It is almost impossible to teach formal English to students who have had no exposure to proper English, either in in spoken or written form. Everything taught in a classroom is undone by the way English is spoken at home and the way it is written by friends and peers. If students are not exposed to proper English while learning to speak, read and write they are unlikely to ever adopt proper grammar as a habit. While there are certainly appropriate times and places for slang and colloquial English, if this is the only type you are regularly exposed to, you see no reason or purpose or use to formal English.

Grammar, new lexics, syntax as well 

Thanks for the invite,

Yasmeen Majeed has provided a good answer that I completely agree with, specially when English is learned as a second or foreign language.