Hi

some clarifications -

I believe both sentences below are correct, just differ in the meaning. (1) The use of "had" imply that he no longer rely on estimation and observations. (2) The use of "have" imply that he is still relying on estimation and observations. Am i Correct?

1) The discovery of error would not have happened if he had not relied on estimation and observations

1) The discovery of error would not have happened if he have not relied on estimation and observations
You cannot use the present perfect (and it would be 'has', not 'have') in the second sentence, because it does not make temporal sense. In addition, the meanings seem strange. You can use these:

The error would not have happened if he had not relied on estimation and observations. -- He relied on e. and o. before the error occurred.

The error would not have happened if he did not rely on estimation and observations. -- He still relies on e. and o.
Do you mind to explain a litte more on what do you mean by - "because it does not make temporal sense"

Also, do you agree that my own explanation for (1) is correct? i.e. He no longer relied on e. and o.

Thanks much.
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Do you mind to explain a litte more on what do you mean by - "because it does not make temporal sense"-- What I mean is that I cannot make sense of the time relationship between the two actions; the verb forms seem impossible together.

Also, do you agree that my own explanation for (1) is correct? i.e. He no longer relied on e. and o.-- Yes. Well, sort of. It says nothing about his present reliance; all it says is that in this case he relied before the error.
One more again,

" The discovery of error would not had happened if he had not relied on estimation and observations"

Why not use had and had for both actions to indicate both actions were in the past...
No, no – you cannot do that in English. This is correct:

The discovery of error would not have happened if he had not relied on estimation and observations.
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