Some of the toughest things about learning German are:
Grammar -The grammar rules in German are different from the other languages. This is because German has a relatively high level of “inflection,” which means that the meaning of the words change depending on their grammatical role.
Pronunciation – One of the hardest thing for German learners is pronunciation. They tend to  mispronounce when they speak because they don’t hold their mouths as tightly shut as Germans do. When Germans speak, they barely open their mouths.
Genders – There are three types of gender (masculine, feminine, and neutral). It is not always easy to identify the gender. Even though sometimes gender can be determined by the word structure, it also depends on some random criteria which is not easy for a learner to understand.