Present Perfect tense is used to describe events which:
- happened in the close past but have influence on present situation.
-- Are you hungry?
-- No, I have just had lunch.
I have lost my keys. Can you help me to look for them?
- take a longer time which continues up to present (the event is not yet finished)
Tom has smoked 20 cigarettes today.
Present Perfect usually does not specify exact time.
Past Simple tense is used to speak about past events which are already finnished. In this tense it is possible to say exactly when something happened:
The World War II started on September 1st 1939.
I am typing. He is shopping.
These actions are happening NOW.
The simple past tense is used for something that occurred in the past and is over.
He went to the store. I wrote a letter.
We don't know when in the past these actions occurred, but we do know that they are over.
The PAST PERFECT is for an action that was happening in the past when something else occurred:
I was writing a letter when the doorbell rang.
"was writing" - past perfect (action ongoing for a while)
"rang" - simple past (action that happened a a single moment)
The simple past is called "preterito" in Spanish and is used similarly to the way it's used in English.
It's called the "passe simple" in French, BUT is NOT used in the same way.
In French, the "passe simple" is used as a (general) past tense only in literary situations - when writing. In speaking, the "passe compose" is used. (I mention this for reference, in case you've studied these languages.)