 Re: the differenses betweeen the usage of present continous and past continous.
Basically, all continous forms are used to describe a temporary on-going event which is "interrupted" by a reference point. The event has started but not finished at the moment of speaking. But it does have a predictable finishing point. Basically, the form of BE tells us the time of the action, and the -ing form tells us that the action is on-going.
The reference point may be the present moment, in which case we use the present form of BE:
Can you call back later - I'm having dinner at the moment
Dinner has started but not finished, and is interrupted by the call- the present moment.
The same thing is happening in :I'm seeing David tomorrow. English considers that the "starting point" of the action is the moment when the appointment was arranged, the end of the action will be when I actually see David. For English, it's therefore a present on-going event just like I'm having dinner, and the "interrupting" reference point is again now, the moment of speaking.
The reference point (and the event) may also be past in time: At ten o'clock yesterday or When the accident happened and in this case the past form of the verb Be is used
I was working in the garden at ten o'clock yesterday.
When the accident happened, I was waiting for the bus |