This question may be somewhat controversial, but, in my opinion, English is a very hard language to learn. When speaking about difficulty, people generally tend to think only about some aspects of the language in question (the ones they find hard, for some reason) leaving out the rest. For example, people often say that Chinese is extremely difficult because you have to learn thousands of different characters, or that Slavic languages are difficult because of all the declensions. However, I think that it’s actually a combination several different aspects of English that makes it difficult.
Some people say that English has complicated grammatical rules, but this really isn’t the case. For example, the verb “to be” has the most forms an English verb can possible have: be, am, are, is, were, was, been, and being (i.e. 8 different forms, while most verbs have just 4).
On the other hand, French être (to be) has 40 different forms! (Just for fun: être, étant, été, suis, es, est, sommes, êtes, sont, étais, était, étions, étiez, étaient, fus, fut, fûmes, fûtes, furent, serai, seras, sera, serons, serez, seront, serait, serions, seriez, seraient, sois, soit, soyons, soyez, soient, fusse, fusses, fût, fussions, fussiez, fussent). Conjugation of verbs is much easier in English than in probably any other Indo-European language.
Although many people complain about the English system of tenses, it is just necessary to understand what the grammatical constructions are supposed to express (without trying to directly translate everything into one’s mother tongue); for example, the phrase “I have been doing something” conveys the idea of doing something continually for a period of time in the past (and usually also continuing the action in the present). Once you understand the concept, it is much easier to apply it to any verb than in most other languages, simply because verb conjugation is much simpler.
Similarly, English has no genders and only three different articles (the, a, an); or five, if you count the stressed variants separately. The form of articles doesn’t change according to the function of the noun in a sentence. Compare the two following sentences in English and in German: