Why are phrasal verbs in English hard to learn?

Why are phrasal verbs in English hard to learn?
【正确答案】：Phrasal verbs in English are hard to learn because, firstly, some very simple verbs can be combined with particles to form quite a number of phrasal verbs. Moreover, most of these phrasal verbs are polysemic.
【题目解析】：参见教材P174。
掌握“Phrasal verbs”知识点。