English learners often have some trouble mastering when to use the present perfect tense and when to use the simple past tense. There are some instances where they are indeed interchangeable, but most often the choice between these two tenses is crucial for conveying the meaning of a sentence.
The present perfect is used when a situation, action, or state is not finished or concluded yet. Let’s look at the following two sentences from Yabla English:
She has lived an extraordinary life of public service.
Caption 36, Barack Obama: on Trump presidential victory
And I lived on a boat for three and a half years.
Caption 8, Great Pacific Garbage Patch: Let's Work for Solutions
In the first example, Barack Obama used the present perfect to indicate that Hilary Clinton has not finished serving the public and will continue to do so in the future. Her life of public service is ongoing. In the second example, the simple past tense makes it clear that the person speaking does not live on the boat anymore. If they used “I have lived” we would know that they are still living on the boat today.
One clue for knowing which tense to use is that certain words like "since," "ever," and "never" are only used in sentences with the present perfect, whereas "ago," "yesterday," "last week" and "last month" indicate finished periods of time that require the simple past tense.
We saw so many incredible places.
Caption 2, New Zealand 100% Pure: New Zealand, Home of Middle-earth
They were the most persistent tigers I've ever seen.
Caption 30, The Marx Brothers: Capt. Spaulding's African Adventures
In the second sentence above, Captain Spaulding means “I’ve ever seen in my life.” Because he is still living, and it is possible he may see tigers that are even more persistent in the future, the situation is considered unresolved and the present perfect is used.
For the next sentence, note that “this project made a huge difference” would mean that the project is finished, whereas how it is written makes it clear that it is, in fact, ongoing:
There's no doubt that this project has made a huge difference
Caption 36, WWF: Making a Difference - Rhino Conservation
In British English, the tenses are more interchangeable. For example, the present perfect is often used when talking about an event that is finished, but happened very recently.
I have just been to Buckingham Palace.
Caption 1, BBC News - Theresa May: First speech as Prime Minister
However, in American English, there are also cases where either tense is applicable. The following sentence is an example in which either tense could be used. This is because the mistake is a finished act, but the situation surrounding the mistake is ongoing.
You really think we made a mistake?
Caption 35, The Big Bang Theory: Consequences Of The Wedding