I've had a good look for it, but I can't find it.

[nelson13]

An example sentence from a dictionary:

I've had a goodlook for it, but I can't find it.

Obviously the act of being unable to find the thing is a past event;

(1)why didn't the editor use COULDN'T?

(2)I remember that in a grammar book the grammarian says when a thing has JUST happened, we can still use the present tense; is this suggestion applicable to the example sentence above?

(3)Is it possible to use COULDN'T? If yes, will there be any change in tone or meaning?

[emsr2d2]

The suggestion is that even though the thorough search took place at some point in the past (I've had a good look), the item is still missing so the person is probably still looking for it even if they are not making much effort to do so. The fact that it is still missing means that we can say "I can't find it".

Also, using "I have had" is naturally followed by the present tense in a lot of contexts:

I have had a lovely day but now I am tired.
I have had a great holiday and I am ready to go back to work.
I have had a lot of great meals this week and I am ready to start my diet.

I would have used "couldn't" in your example had it started "I had a good look" (not I've had")

I had a good look for it this morning but I couldn't find it. I will have another look later.

[nelson13]

The suggestion is that even though the thorough search took place at some point in the past (I've had a good look), the item is still missing so the person is probably still looking for it even if they are not making much effort to do so. The fact that it is still missing means that we can say "I can't find it".

Also, using "I have had" is naturally followed by the present tense in a lot of contexts:

I have had a lovely day but now I am tired.
I have had a great holiday and I am ready to go back to work.
I have had a lot of great meals this week and I am ready to start my diet.

I would have used "couldn't" in your example had it started "I had a good look" (not I've had")

I had a good look for it this morning but I couldn't find it. I will have another look later.

Thank you very much for the detailed answer. It's very interesting.

After a discussion, international students in lectures express their opinions always by saying:

'We thought that....'

Obviously they think what they were discussing is in the past, so they use the past tense; but isn't it more logical to say WE THINK THAT...? Their opinions can be permanent truth and unchanged when they air the opinions....

[5jj]

After a discussion, international students in lectures express their opinions always by saying:

'We thought that....'

I have heard students say both 'we thought' and 'we think'. Both are possible. The first places the thought at the time of their discussion, or the end of it; The second is referring to their present thought.

[emsr2d2]

That's another example of mixing the two tenses.

"We've had a long chat about it and we think the answer is twelve."