"present" vs "present perfect"

Question

There are 2 sentences that I'd like to prepare.

1. I have heard recommendations from doctors.

2. I heard recommendations from doctors.

I think sentence 2 is wrong because 'recommendations' is plural and that is compatible with present perfect tense used to talk about several different actions which have occurred in the past at different times. However, past tense is more compatible with an action that occurred once before. So, I think it would be more suitable to put sentence 2 like a following.

I once heard a recommendation from a doctor a couple of days ago.

I'd just like to make sure that I use each tense properly.

So, could you let me know if I'm right to think this way?

Answer 1

I have heard recommendations from doctors.

This tells us something about who you are now. You are someone who has heard recommendations in the past. It describes your present state. It is a fine sentence.

Q: How do you know so much about the danger of eating margarine?

A: I have heard recommendations from doctors. I have read books. I have done some laboratory research on my own.

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I heard recommendations from doctors.

This tells us about something that happened in the past. You heard something. It is a fine sentence. There is no problem with using past tense for plural events.

Q: Why did you quit eating margarine?

A: I heard recommendations from doctors. I had my cholesterol tested hundreds of times.

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I once heard a recommendation from a doctor a couple of days ago.

This is the only one that doesn't work. When you begin a sentence with "I once ...." you have announced that the rest of the sentence will be framed at some unspecified point in the past, and that the precise date probably doesn't matter. One would never finish the same sentence by specifying the date so precisely.