"I saw one item in the museum that I will never forget."
I am a foreign student learning English. I learned that "will" should be "would" if the sentence is written in the past tense like "I thought that he would come back". However, in the above sentence, will is used. I wonder the reason.
The fact that you "will never forget" is still true in the present; nothing between your time in the museum and the present time could have altered that fact. So typically you would not backshift the "will" into the past tense here.
There are cases where you would need to backshift:
I saw something in the museum that I would never forget for the next three years.
In that sentence, you need to move the "will" into the past tense, because the statement "I will never forget it for the next three years" is (presumably) no longer true.
The meaning of the sentence is this: I saw something in the museum, and I will never forget that thing.
If you use "would", a past verb, it means that "never forget" only happens in the past. That means, you may have already forgotten the thing in the museum. This doesn't make sense because of the word "never", which extends into the future. The intent here is "until the end of my life", and obviously if the person is still talking, their life isn't over.
The only time it would be natural to use "would" there is if it's someone who is near death looking back on their life, noticing that after seeing that item in the museum, they would never forget it.
