English is considered difficult due to a number of factors. First, many of the rules in the English language have so many exceptions that the rule is difficult to know when to apply and not to apply. Another thing, unlike most European languages, there are very few words that hold a gender. You know what endings for plural to put on most European words, for example, based on the word's gender, while, with the exception of obvious words such as "mother" or "girl", most words in English do not have a strict gender and it can seem a bit random what to put on the end for something for plural. For example, the plural or "goose" is "geese" but the plural of "moose" is "moose"....it can mean one or more than one. Also, English pronouns that signify gender: "he", "him", "her", etc can be very confusing for foreigners and are probably the biggest problem most foreign language students have, and the English language is also steeped in figurative language that is based on the culture that many find just outright confusing.

I'm writing this as a 20+ year English teacher who's spent the last two years scoring the TOEFL (test of English Fluency) that a lot of non-native English speaking high school and college students take to gain entry in to an English speaking school. If you have any questions you'd like to pose to me, feel free. I don't have all the answers but will be happy to help if I can.