eslHQ - what are the diffrences between the usage of present simple and the past simple

Hi Kamal,
The names "present" and "past" simple are actually misleading, because the present simple isn't necessarily present and the past simple isn't necessarily past. Can we forget them and talk instead about first form verbs (go, live, see etc) and second form verbs (went, lived, saw)?

First form verbs

First form verbs basically describe an event or action which is permanently true - it was true yesterday, it's true today and, for the moment, there is no expected or decided end in sight :

Water boils at 100°C
I live in Italy
If you press this button, the light comes on.
In the film Casablanca, Ingrid Bergman walks into Humphrey Bogarts bar ...
To get to the centre of town you go down Holton Road ...
The train leaves at eight o'clock.

It's always the same : any time you heat water, it boils at 100, every time you watch Casablanca the same thing happens, the train leaves at 8 every evening - even if it may only be tomorrow evening that we're interested in. There are only two cases where the use seems to be extended : the first follows from What time does your train leave? It leaves at 8pm. as I said, although we may only be interested in tomorrow, it is actually a regular, timetabled event which is permanently true. But we also use the form for timetabled events which are not permanently true : When does the meeting start? It starts at 3pm. This would be used even if the meeting was not a regular event but a one-off.

The other extension of the use, is the use after conjunctions (when, before, after, if, until, unless, as soon as etc) to express future events :

Let's have a coffee before the meeting starts.
If you see John at the meeting, can you give him a message.
When it stops raining, I'm going for a walk.

Second form verbs

Second form verbs basically describe events which are "distanced" from here and now reality. If something is "distanced" from now then it is a past event - it has no connection with the present time :

I went to Turkey last year.
I saw John yesterday.
I lived in Paris from 1990 to 1992

This is obviously the most common use of second form verbs - and the reason why they are usually called "past". But second form verbs aren't only used to express past time. Another way that an event can be "distanced from reality" is if it is hypothetical. And second form verbs are also used to express hypothetical events .

I wish I had John's phone number (reality : I don't have it)
If I knew where the keys were, I could give you a lift in the car. (reality : I don't know where they are so I can't give you a lift.)

These are the two main uses of second form verbs. There are others which are less common and less important, but they all express this idea of "distance from here and now reality"