16.We started to write to each other three years ago.-> We have writen to each other for three years

17.The last time I went to the museum was a year ago. -> I last went to the museum a year ago 

18. I have never played golf before. -> This is the first time I have ever played

19.This is the first time I have seen a spacecraft. -> I have never seen a spacecraft before 

20.John used to write home once a week when he was abroad. -> John often wrote home once a week when he was abroad