[英语达人请进]新概念2lesson14的一个问题

[英语达人请进]新概念2lesson14的一个问题
文章内容
I had an amusing experience last year.After I had left a small village in the south of France,I drove on to the next town.On the way,a young man waved to me.I stopped and he asked me for a lift.As soon as he had got into the car,I said good morning to him in French and he replied in the same language.Apart from a few words,I do not know any French at all.Neither of us spoke during the journey.I had nearly reached the town,when the young man suddenly said,very slowly,’Do you speak English?’ As I soon learnt,he was English himself!
请问最后一句话,as i soon learnt,.
为什么这里是learnt?我很不解