1.to put up → to be put up
宾语the new classroom building和宾补动词put up之间为被动关系,所以用其被动结构.
want sth to be done意思是“想要某事被做”.
2.to come → come
定语从句you saw come in with a big traveling-bag中,（被省略的）关系代词whom代替先行词the stranger,充当saw的宾语.
从句中有感官动词saw,whom/the stranger为宾语,宾补用动词原形（即不带to的不定式）come.
如果用coming,则翻译为“你看见的那个正带着个大旅行袋进来的陌生人”,不太通顺.
如果用come,则翻译为“你看见的那个带着个大旅行袋进来的陌生人”,更为准确.