这句话的确是一般现在时,所以过去式offered显然是错误的
since有三种词性：
用作介词,表示自从,常与完成时连用
I haven't written home since Christmas.
用作副词,表示从那时,一直到现在
Things have not changed much since.
His first book was a great success and he has been famous as a writer ever since.
He got a job with the firm in 1970 and has worked there ever since.
在这里是用作连词,引导的是原因状语从句,表示因为、既然、鉴于……
Let's go for a walk,since the rain has stopped.
Since we are young,we shouldn't be too afraid of making mistakes.