Learn Swedish: Jag har inte hört knastret på flera år. - I haven't heard the crackle in years.

This ties into the main rule for this lesson! When talking about how long something has not happened (duration in a negative sentence), Swedish uses på. If this were a positive sentence (like "I have heard it for years"), you would use i. Because of the word inte, the duration requires på.

Why is the word for "the crackle" spelled knastret instead of knasteret?

The base word is ett knaster (a crackle). When an ett word ends in -er, -el, or -en, it almost always drops the final -e- in the stem before adding the definite ending -et. This makes it flow better when speaking, so it shrinks down to knastret.

The English translation says "in years", but doesn't flera år translate to "several years"?

Yes, literally it means "for several years". While English speakers often drop the adjective and just say "in years", Swedish prefers to keep a descriptive word. You will commonly hear på flera år (for several years) or på många år (for many years) to express this exact idea.

Just to review, why is inte placed between har and hört?