it is not long before they are so accustomed to swimming.急

It is not long before they are so accustomed to swimming.
没用多久他们就对游泳习以为常了.
be accustomed to （doing）sth,习惯于做某事
这里的to并不是不定式,而是介词,在介词后的动词要变为动名词,做介词的宾语.