I bought a Cherry mechanical keyboard yesterday. Cherry is a German keyboard-making company. I had a chance to use a Cherry keyboard before. One of my coworkers used a Cherry keyboard, and he let me use it for some time. It felt very nicer to type on than my membrane keyboard. I hadn't understood why people buy such expensive keyboards, but after using his, I realized why people buy them.
My native English-speaking friend told me that I can't say "I hadn't understood" because it sounds like a discrete action and thus sounds very weird to him.
I used the past perfect tense to refer to a time earlier than the time when I had a chance to use his keyboard. Could you grammatically explain why it is wrong?