Technically, where the ‘if’ can be interchanged with ‘whether’, then it should be a
e.g. I don’t know whether/if it will be fine tomorrow - Dwi ddim yn gwbod a fydd hi’n braf fory
BUT you’ll hear os, being the primary Welsh word for if, and due to the influence of English, often being used in speech in both cases - where the if = whether and where it it doesn’t (in both N and S).
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