have to 后接动词原形 用practise 是对的.
关键是practise 后该用不定式还是动名词.一般而言,动词+to do sth 指的是具体的某件事情或还未做的事情,而动词后+doing sth表示一类事情或一直在做的事情.
改为 You have to practise speaking English as much as you can.