I am not sure whether the past perfect (1) is mandatory in cases like the following. I wonder if in everyday speech the versions with the simple past (2) would be just fine. I think native speakers often use (2) instead of (1), so the past perfect is not needed.

1)Damn, it doesn't work! I thought I'd found a solution, but this is totally useless!
2)Damn, it doesn't work! I thought I found a solution, but this is totally useless!

1)I thought your mother had died, that's why I was trying to avoid talking about her.
2)I thought your mother died, that's why I was trying to avoid talking about her.

1)I knew Jack had given her that money, so I was trying to find it. I felt she'd probably hidden it in the kitchen.
2)I knew Jack gave her that money, so I was trying to find it. I felt she probably hid it in the kitchen.

Hi Kooyeen

Yes, I'd say that the past perfect would often be omitted in informal English. But I'd also say that this depends very much on the person. I remember being pleasantly surprised the other day to hear my 17-year-old niece use the past perfect in a sentence in a very informal family setting. And they claim kids don't know how to talk anymore... Emotion: stick out tongue
Whoa! LOL, I didn't think the past perfect was so bad in informal English. I mean, if you noticed it and you were surprised, it must mean it even sounded kind of odd to you! Well, what if she'd used "whom"? Y'woulda pass out? I would... Emotion: stick out tongue

Anyway, I see. Thanks!
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Hey, I was delighted to hear my 11-year-old say "If it were..."
Hey, Kooyeen, I didn't say it sounded odd! Quite the contrary. It sounded completely normal. Don't forget I've been an ESL teacher for umpteen years. As a result, my ear is now highly trained to pick up on and take note of these sorts of things. In short, I tend to pay much closer attention to grammar usage than the average person does. Emotion: smile

If my niece had used 'whom', on the other hand, that might indeed have raised at least one of my eyebrows. Emotion: wink
Ah, ok. It was due to a "professional deformation" then! (not sure it's the right term, but that's what we say in Italian). Emotion: wink I see, thanks.
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A "professional deformation"? Emotion: surprise You have such a way with words, Kooyeen! I'd never thought of anything about myself as being deformed before. Emotion: stick out tongue

I suspect the English equivalent of your Italian expression might be "occupational hazard".
FYI. I haveheard the expression 'professional deformation' in English. Emotion: smile