1. It's past tense. you found (past) that it damaged (past) the clothes. 2. This one contains both past, and present tense. You found (past) that it damages (present) clothes. The second sentence implies that when you use the item, it damages clothes. The first sentence only says that you used the item, and it damaged your clothes. The first sentence doesn't imply that if, you used the item again, it would damage your clothes again, unlike the second sentence, which does.

1, I found that it had seriously damaged the clothes Past Simple + Past Perfect 2, I have found that it seriously damages the clothes incorrect sentence