Once Newton started to do experiment he forgot about the time.

只要牛顿开始做试验,就忘记时间了.
once就是只要...就...的意思
once Newton started to do experiment就是时间壮语~

一旦牛顿开始做试验,就忘记时间了.
时间状语从句

(有)一次,牛顿开始做实验,他忘记了时间.