I thought

[Unregistered]

How are you?

Is there any difference between (1) and (2)?

1, I thought I left my book on the desk, but it isn't there.
2, I thought I had left my book on the desk, but it isn't there.

[danijela74]

Of course there is a difference. Your second sentence is gramatically incorrect. It should go like this:
I thought I had left my book on the table (prior to doing something else ), but it wasn`t there.

[Steven D]

Of course there is a difference. Your second sentence is gramatically incorrect. It should go like this:
I thought I had left my book on the table (prior to doing something else ), but it wasn`t there.

I wouldn't say so. It's not necessary to show that an action expressed with the past perfect occurred prior to something else in the same sentence in order for the sentence to be correct. It could be understood within the context of the conversation.

[Steven D]

How are you?

Is there any difference between (1) and (2)?

1, I thought I left my book on the desk, but it isn't there.
2, I thought I had left my book on the desk, but it isn't there.

Number 2 is a sure indication that you left your book on the desk and then did something else even though this is not written in the sentence.

Apart from that, they essentially mean the same thing. Either way you did something after you left your book on the desk.

2, I thought I had left my book on the desk, but it isn't there. [ before I left my office ]

1. You left your book on the desk. 2. You left your office.

It's not necessary to include the part about leaving your office in the sentence that uses the past perfect for it to be correct.

[danijela74]

I know it doesn`t have to be mentioned in the sentence, that`s why I put it in brackets, just to confirm on the implication, but the part I corrected is the one " It isn`t there" with "It wasn`t there". That is the part I thought was not correct.

[Steven D]

I know it doesn`t have to be mentioned in the sentence, that`s why I put it in brackets, just to confirm on the implication, but the part I corrected is the one " It isn`t there" with "It wasn`t there". That is the part I thought was not correct.

Oh, I see. Well in that case, I would say there is still no error.

The speaker could say "but it wasn't there" after having looked. In this way he/she would be acknowledging that it was not there at the time he/she looked. The speaker could also say "but it isn't there". In this way he/she would be acknowledging that it is still not there as he/she is speaking. Both are correct. It depends on the how the speaker views the situation and where he/she is while speaking. It's possible to shift from the past to the present in the same sentence.