Mercedes __ (work) as a teaching assistant since she arrived here 2 years ago.
Mercedes HAS BEEN WORKING as a teaching assistant since she arrived here 2 years ago.
I __ (try) to finish my assignment all through the plane journey, but it was too turbulent.
I WAS TRYING to finish my assignment all through the plane journey, but it was too turbulent.
The rearranged lecture __ (take) place next Friday at 5.00 p.m.
The rearranged lecture IS TAKING PLACE / WILL BE TAKING PLACE next Friday at 5.00 p.m.