I think the brain is designed to free up memory holding information that is not useful. Memory associated with a technical term like phrasal verb seems to be so

I think the brain is designed to free up memory holding information that is not useful. Memory associated with a technical term like phrasal verb seems to be something it would garbage collect. The brain will remember the term while it’s useful — to get a good grade on a test. After that, for most (almost all?) people it is useless to remember.