1 we can send and receive email very quickly thanks to the internet                      enable sb to do 
=> the internet enables us to send and receive email very quickly
2she was working as a volunteer in south africa. She met her old friend                             QKĐ + WHEN + QKTD
=>she met her old friend  when she  was working as a volunteer in  south africa
3 she stared teaching E here when she graduated from university in 2013             SINCE = từ khi 
=>she stared teaching E here since she graduated from university in 2013