Пжл.. помогите )))

Вставитьположительные или отрицательные формы глагола can , to be able в правильном времени.

In very early times all the people on the Earth_________speak the same language.They lived together and ______ understand each other.If we all get together we________building a great city/If we________do it everybody will be happy.They________work really fast.Nowadays people ________spea many languages but most of them_________speak English.