To protect themselves from the icy water,they had to wear special rubber suits...这句是不定式做目的状语从句吗,还是目的状语To protect themselves from the icy water
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是目的状语,“为了保护……”
======以下答案可供参考======
供参考答案1:
是目的状语，“为了保护……”
供参考答案2:
have to 是短语吧，不可分割，约等于must的意思。貌似不是不定式。