I have made up the sentences below.
Suppose that I'm holding a watch in my hand, showing it to my friends and I said to them:
1a)
This watch is the fourth one I got for my collection.
1b)
This watch was the fourth one I got for my collection.
Now suppose that my friends and I are watching a very old movie at a movie event and I said to them:
2a)
This movie is the fourth one ever made in the world.
2b)
This movie was the fourth one ever made in the world.
I would probably use is if I had only just bought the watch, was if I had been collecting for some time and had bought other watches since.
Similarly, was seems appropriate for a very old movie.
I would use is in both cases, because it was still true at the time of speaking and it's still true now, that it's the fourth watch that you bought and that it's the fourth film ever made.
That said, there's nothing wrong with using was as well, because it was also true bought the watch and the film was made before your talked about them.
Is/was statements re an ended situation or a continuing one.
In Speech
A) The is/are/am works if the situation continues to be true.
B) The was/were works also, if it does not.
Context is crucial: A) I told them, "This watch is the fourth one I got for my collection." [general statement about which number watch this is for you]
B) I told them, "This watch was the fourth one I got for my collection." [but now you have more watches]
Indirect speech A1) I told them this watch was/is the fourth one I got for my collection.
The same idea as above applies.
reported speech and past versus present tense
Assume that Kim has blue eyes, and that Tom knows that. And then Tom tells me,
"Kim has blue eyes." Then I can tell you,
"Tom told me that Kim had blue eyes." which has backshifting. Or I can choose to not use backshifting,
"Tom told me that Kim has blue eyes." Both versions are fine. Note that Kim's eye color is assumed to be a permanent sort of thing. [bolding mine]
