How do you explain the subtle difference in the usage of the present perfect tense and the simple past tense in English ?

We do not use the present perfect with an adverbial which refers to past time which is finished. But we can use it to refer to a time which is not yet finished. The present perfect is formed from the present tense of the verb have and the past participle of a verb. The present perfect continuous is formed with have/has been and the -ing form of the verb: We use the present perfect tense for something that started in the past and continues in the present: i.e. They’ve been married for nearly fifty years. We normally use the present perfect continuous for this: She has been living in Liverpool all her life. It’s been raining for hours. We can use for something we have done several times in the past and continue to do: I’ve played the guitar ever since I was a teenager. We often use a clause with since to show when something started in the past: They’ve been staying with us since last week. When we are talking about our experience up to the present: We often use the adverb ever to talk about experience up to the present: My last birthday was the worst day I have ever had. And we use never for the negative form: Have you ever met George? Yes, but I’ve never met his wife. We also use for something that happened in the past but is important at the time of speaking: I can’t get in the house. I’ve lost my keys. Teresa isn’t at home. We use the present perfect of be when someone has gone to a place and returned: Where have you been? I’ve just been out to the supermarket. But when someone has not returned we use have/has gone: Where is Maria? I haven’t seen her for weeks. She's gone to Paris for a week. She’ll be back tomorrow. We often use the present perfect with time adverbials which refer to the recent past: just; only just; recently; Scientists have recently discovered a new breed of Lions.

Hi! 

Aside from the "has + verb past simple" and "have + verb past simple" formulas which, I believe you already mastered,  being specific with time, and the use of some adverbs of degree play big roles here. 

Consider these examples and hope we agree with the same correct thoughts. 

1. I / We / They / You       have (just) finished editing this sentence.

2. He / She / It     has (just) stopped running.

3. Have you (ever) watched      Sound of Music?

4. I have (never) watched         Sound of Music.

5.Uh oh, Johnny has taken       IELTS exam for the fifth time already!    (repetitive action)

 

Some of these may still be very vague for new ESL, EFL TESOL, ESOL etc... learners. But eventually they will catch up with the usage as you drive them to understand.

Enjoy teaching1

 

The present perfect is used when there is a relation between the past and the present. This relation can be the continuity of the action or the presence of its effect in the present. On the other hand, the past perfect is used when the action happens in the past and there is no mentioning of a relation with the present time, e.g.,

I have lost my keys. (the message is that I lost them and I haven't found them yet)

l lost my keys this morning. (there is no indication whether I have found them or not, what's more important to the speaker here is that this event happened and it happened this morning)

We use this tense when we want to talk about unfinished actions that started in the past and continue to the present. Usually we use it to say 'how long' an action or state has continued with 'since' and 'for'. Often,

we use stative verbs in this situation:

We use 'since' with a fixed time in the past (2004, April23rd, last year, two hours ago). The fixed time can be another action, indicated with the past simple (since I was at school, since I arrived

We use 'for' with a period of time (2 hours, three years, six months