Complex sentence: Cohen's results supported the regression hypothesis and his last-learned-first-forgotten thesis, that some things, which are learned the last are the first to be forgotten, when the learner has no input of the target language anymore.
Split into simpler sentences: Cohen's results supported the regression hypothesis and his last-learned-first-forgotten thesis. It supported the idea that some things, which are learned last, are the first to be forgotten when the learner has no input of the target language anymore.