Shw mae, Nicola? And welcome to the forum.
It sounds like you’re doing well to me!
You’ve got to the first of many confusions, and are getting past them: that is the main (posibly the only?) part of learning anything new, and especially a language. So that’s two big ticks already.
I won’t answer your specific questions, as everyone else hsa beat me to that (I blame being on holiday!) but I wanted to welcome you, and to point out a pretty important feature of the SSiW course:
What you are learning is not some mythical “southern dialect”, but quite specifically “MY southern dialect”. There are lots of people in south Wales who speak like me, and lots (probably more) who don’t. What you have learnt is very likely to me more “standard” than my daiect, and equally valid, so don;t feel that you have learnt anything wrong - you are just already a bit more flexible than a completely fresh learner.
It sounds like you’ve found that little bits of unexpected Welsh slip to the surface from time to time - this is a great thing to happen, even (especially!) when it’s slightly different to what you’re learning with us, because it means that you already have two ways of saying some things. That may seem a bit confusing to start with, but it will help you immensely when you start hearing other Welsh speakers talk, because your experience is already greater than just the course material. That’s a pretty handy bonus for you.
I hope you’ve arleady found that there are plenty of people on this forum who are willing to help you over the obstacles that you will find in your way as you learn. Remember that most of the advice that you get here will be from people who have already come across the same confusions and obstacles as you are finding, so their advice is the very best kind. I hope that quite soon, you’ll be in their position for some newer learner - helping others is a great way of helping yourself to understand stuff, and think about stuff that you may nto have considered before. As a native speaker myself, I’ve learnt loads from people on this forum, so it really is a treasure trove of advice and information.
And don;t worry about not remembering full sentences. That’s something taht will develop as you get usedf to the course. You seem to be comping well enough with it at the moment, so just keep doing what you’re doing - it certainly seems to be working for you.