English speakers usually have a hard time understanding the difference between fuit and erat. That's not really shocking. They're both translated the same in English, and the distinction is both subtle and not one we make in our language.
The crux of the issue has to do with the perfection or imperfection of the verb. When we use fuit, we mean that the state of being described has come to an end, i.e. is perfected, whereas erat makes no such claim.
Suppose I put my keys on the table, and then later, someone moves them without my knowledge. I walk into the room, and I see that my keys are not on the table. They were. And now they aren't. I might say, Ubi est clavis mea? Fuit in mensa!
Now suppose I don't know my keys have been moved. Someone comes to me and asks where my keys are. As far as I know, they're on the table. Now, I might say, Clavis mea erat in mensa.
The difference is that in the first case, the keys are no longer on the table. They were, and now they aren't. They are done being on the table. The act of being on the table is perfected, so I say fuit. In the second case, I have no reason to believe my keys are not on the table. They were, and to the best of my knowledge are still there, continuously carrying out the action of being on the table. There, I use erat.