Can you begin a sentence with therefore?

Yes, Words like "since" or "because" can begin a sentence. The following sentences are examples that begin with "since":

• Since I haven't heard back from you, I will assume you're not interested.
• Since you've been gone, a lot of things have changed around here.

当然可以，例如：因此，我们需要更加努力地学习和工作。