Yes, there are a number of ways to say things in past tense. This thread (and the video linked into it) might help Wnes i ddim/ Do'n i ddim
As for medru, yes, there are long and short forms in the present too, and yes, you can use “dwi’n medru”.
Liciwn is a conditional form (I would like), but you can say dwi’n licio (I like), and nes i licio (I liked) for present and past.
Don’t worry about getting all the forms mixed up at this point - it’s a lot for anyone’s brain to process and takes time to slot into place, but it will do eventually. Even if you pick the ‘wrong’ one, it’s likely you’ll be understood, and mistakes are great learning tools anyway so don’t be afraid to make them! 