Except…in Southern course it’s actually “Ma dal rhaid i fi”!
I guess that’s a regional preference.
However for @sianelagreen doubts: I believe that SSiW works much like learning straight from people rather than in school. Like mother tongue. I can’t explain why use one thing here and the other thing there, but I know which one is right.
And I can tell it works!
But of course whenever one feels the need to understand better, it’s fair to ask and you always get answers here (you can also try a search in the forum, to start with or have a look at this thread:
https://forum.saysomethingin.com/t/tiny-questions-with-quick-answers-continuing-thread/3153