have to be built是被动语态,其主动语态为：have to do.
动词不定式的被动式：to be done
have been built是被动语态的现在完成时,表示已经被建成．
eg.The tall buildings have been built.那些高楼已经建成了．
而have to be built是不得不被建的意思．
have to do不得不做...