一般过去时:强调的动作的过去,yesterday ,the day before yesterday ,an hour ago
现在完成时:强调过去发生的动作对现在造成的后果,for three years since 2008,already
过去完成时、过去完成进行时:多数用于宾语从句中 They told me they had been to the USA many times.They said they would get to the USA the next day.