Marcos Paulo Silveira Vilaça wrote:Don't call us before because we will be attending a lecture by then
Is it correct?
It is not correct, and has nothing to do with passive voice.
The word "before" is a preposition that requires an object that expresses a certain time or date.
The expression "by then" indicates something that happens
after a specified time, the opposite of before.
These examples would make sense:
Please don't call us before
11:00 because we will be attending a lecture
until then.
Please don't call us
after 10:00 because we will be attending a lecture
by then.
"Make it go away, Mrs Whatsit," he whispered. "Make it go away. It's evil."