"I've commented on your story" hints that in the past, back in the day, you have commented on her story at some unknown, random point in time. This could be used like: "I've commented on your story before, and I don't have many more changes to make." "I commented on your story" hints that in the past you commented, and it might have even been fairly recently. This would preface something like: "I commented on your story, but I'm not sure if I was helpful or not."
"I've commented on your story" is correct present perfect. "I commented on your story." is correct simple past. Each has its use. Which is best depends on the context. "As requested, I've commented on your story. Please find attached a copy of your original file marked up with my comments." "I've done lots to support your writing career! I commented on your story. I mailed it for you. I even paid for the postage stamps!"
Hi vanilla16 Both examples are correct. You are merely expressing the message in two different past tenses. I commented on your story is written in simple past tense. I've commented on your story is written in present perfect tense, which, despite the name, is still a past tense! (much like what the poster above said, if only I had been sufficiently alert to read it first! BTW "had been" is past perfect progressive tense.
They are the names of the different tenses and aspects. There are descriptions with helpful diagrams here.
And lots more don't know when to use an apostrophe (Sorry, we all fall victim to Muphry's law from time to time)
May I ask one more question? Both of these sentences are correct? I have already completed my assignment. I already completed my assignment
No, the difference is definitely subtle between past tense and present perfect, especially in this example. But there is a difference. The first sentence uses present perfect tense for an action that has occurred before the present time. The second sentence uses past tense ti denote an action in the past. A more obvious example: I flew to Europe. I have flown to Europe. The first is more definite, and speaks of a specific instance, but the second means that at some indefinite past time the speaker has travelled to Europe by air. Google present perfect tense for a more detailes explanation. In some contexts, there is little difference between present perfect and past tense semantics, but in other cases the difference is very significant.
Both are grammatically correct. Because simple past usually refers to a specific time, and "already" is vague about the actual time, "I have already completed my assignment" would probably be better, but you will hear both.
One instance where only one of the tenses can be used and not the other is: "I completed my assigment at 3 o'clock yesterday." You could not use present perfect then, because we're talking about a specific point in past time. "I have completed my assignment at 3 o'clock yesterday." is incorrect, and I'm sure you can hear it. Present perfect always points at an undefined (and irrelevant) moment in the past, and the fact that the assignment indeed has been completed by now is in focus.
