The important thing you should is to make it come true.这句话是什

should 后面加do,The important thing 可以加that/which,因为先行词在从句中作do的宾语.
The important thing you should do （that）is to make it come true.是错的
因为do的宾语是thing,不用再加宾语了.
The important thing 主语 you should do 定语 to make it come true 表语
you should do是句子作定语,所以是定语从句