英语为什么过去式的句子中用现在进行时?

英语为什么过去式的句子中用现在进行时?
2.Today was Sunday.Li Ming and Zhang Hong were going to the library.They were talking and laughing happily.An old woman with a basket was walking in front of them.Suddenly the old woman fell over and the apples in the basket dropped on the ground.They ran up to her as quickly as they could.Li Ming helped the old woman stand up.Zhang Hong helped to put the apples into the basket.Luckily there was nothing serious with her.They decided to send the old woman home together.The old woman was so thankful that she couldn’t say a word.What kind children they were!We should learn from them.