*we use for and since to say how long something has been happening.
1- we use for + a period of time (two hours,six weeks etc.):
- I have been waiting for two hours
- I have been waiting for two hours
two hour ago ---------> now
- Sally has been working here for six months .(not since six months)
- I haven’t seen tom for three days .( not since three days )
- I haven’t seen tom for three days .( not since three days )
2- we use since + the start of a period (8 o’clock ,monday,1999,etc.):- I have been waiting since 8 o’clock .
8 o’clock---------> now
- Sally has been working here since april.(from april until now)- I haven’t seen tom since monday .(from monday until now )
