I learnt Mandarin Years Ago
I learnt Mandarin when I was in college, but it was just for about two semesters. It was not enough to make me fluent in it Mandarin. Because As I didn't continue the learning process, I forgot many vocabularies of the words I learnt. or "I forgot much of the vocabulary".
I Learnt Mandarin Years Ago
I learnt Mandarin when I was in college, but it was only for (just about) two semesters/terms and (. It was) not enough to make me fluent, (in Mandarin.) (Because) I didn't continue the learning process (,) so I forgot much of the vocabulary (many vocabularies) I had learnt.