In the old days, I would study several languages besides English: Chinese, Japanese, French, Spanish, Korean and even Thai. I learnt a little of each, but none well enough to satisfy myself. I realized I ought to study only one language, but carefully. So, my busyness led me to quit all of them but English. At times, my memory amazes me. I still remember a lot of Chinese and Japanese grammar. I sometimes still read French. It makes me happy not to be as forgetful as I thought. "My old days" is not wrong, but "the old days" is a more common phrase. Say "well", not "good" because you need an adverb to modify the verb "learnt". You are compounding your sentences too much. It is no sin to write short simple sentences. Your last sentence consists of two independent clauses that are separated by nothing - not even a comma. You are not allowed to do this. You have three choices: either turn it into two sentences "It makes me happy. I am not forgetful as I thought" or, decide which sentence will be the main clause and which will be a subordinate phrase "That I am not as forgetful as I thought makes me happy" "Making me happy, I remember more I would have expected." "Happily, I am not as forgetful as I thought" or, make a compound sentence "It makes me happy, because I am not as forgetful as I thought." We say "grammar", not "grammars" in the U.S. I don't know about the U.K.

* I used to learn a lot of languages besides English: Chinese, Japanese, French, Spanish, Korean and even Thai. I learnt a little of each but none of them /none of the languages was good. I realized I should learn only one language carefully /well because I was busy, I quit /stopped all of them but /except English. From that time on /From then on, at times, I was /*have been amazed that I still remember lots of grammar and vocabulary of Chinese and Japanese, and I can read French sometimes. It makes me happy that I am not as forgetful as I had thought. *the first phrase is not necessary *if you are still amazed about remembering then present perfect