I was walking along the street?when?I heard my…（并列句）备注：when 为并列连词，相当于at this time或者at that time。所以为并列句I was about to leave my office when the telephone rang.我正要离开办公室时,就在这时忽然电话铃响了。When/While/As?I was walking along the street, I…（复合句）备注:when 为从属连词,相当于?at or during the time that当....的时候,在...期间，所以为复合句。I loved history when I was at school.我上学时喜欢历史。备注：when在以下几种情况被认为是并列连词引导并列句【1】be doing sth +when? I was cooking when I heard her knocking at the door.??我正在做饭，突然听到她敲门的声音。【2】be about to do sth +when?? We were about to start when it began to rain.??我们刚要动身天就开始下起雨来了。【3】be on the point of doing sth +when?He was on the point of leaving?when someone knocked at the door.??他正要走就有人敲门。