Hi,everyone. I have a question about the tense. 'By the time we'd walked five miles，he was exhausted.' Usually I was told that when 'by the time' appears in the sentense, it followed by past tense and the main clause's tense is past perfect tense. Just like 'by the time when got to the bus station,the bus had left.' So I am confused. I don't know how to use right tense when I write. Please help me. Thank you.