What is the difference (in terms of meaning) between the simple past and the present perfect/ past perfect?
Past Simple is a tense used to inform about actions which took place and were completed in the past.
I ate a breakfast. - this sentence informs about a completed action in the past.
Present Perfect is a tense used to inform about a completed action in the past which has its influence on present situation.
I have eaten a breakfast. - this sentence not only informs about a past action, but it also indicates the action's influence on present situation. I have eaten a breakfast could be an explanation of answer "I am not hungry" For example in a situation like this:
A: Would you like anything to eat?
B: No, thank you. I have (just) eaten breakfast.
Past Perfect is used to indicate which of 2 past actions was compleated earlier than the other.
After John had written his homework, he went out to play football.
In this sentence we say about 2 actions in the past. The earlier one (writing homework) is expressed in Past Perfect, while the later (going out) is written in Past Simple