Chen (work) in Singapore, since he (leave) university.
has been working/left
Alexamder takes university life very seriously. He(study) here for six months and still (not go) to a single party.
has been studying/hasn't been
Maria (come round) to dinner last night.She (start) telling me her life story, while I(make) the salad and (continue) telling it during dinner.
came/started/making/continued