The most common cause of the problems faced by a learner of the English language as a second language is existence of the inherent structure of their first language or mother tongue that they are exposed to since the first day. As such the problems that are faced by the students trying to learn the language are-
Understanding the grammar structure of the new language.
Pronounciations or the inability to understand or comprehend new words.
Most often students use the translation method to understand the second language, in this case English
Again they think of the reply in their mother tongue first and then form a reply by translating.
Vocabulary becomes a huge problem.
The above mentioned problems are just a few of other problems but in my opinion these are the major ones faced by students. the hardest for a teacher in this case is to actually create a new structure of the English grammar while trying not to dismantle the exisiting language structure of their mothertongue. The student should be able to use both the langauges with ease. Again it is fairly easier in the case of children.