What actually is a 'worksheet'? I have tried to find a Swedish translation but ended up with everything to 'an Excel document' to 'sort of any piece of paper at work'. If i ever get to Wales and someone asks me for a 'taflen waith' I have to know what to do. :)
In school it could be an exercise on paper that a pupil needs to complete, maybe by writing in blank spaces. This seems to be the meaning here. As you say, it could have a wider meaning depending on the context, including the Excel document or a set of instructions.
I would go with a sheet of work in a school setting. At least, that's what it is in the US. I ask my kid, "do you have any homework?" she says, "Yes, but only one math worksheet."
Generally, it's a paper with exercises on it to practice something you are learning at school. Also, you can use worksheets to figure out information belonging on a tax form. Those are not an official part of your tax return but something which is intended as an aid for you in completing the tax forms in the return.
Taflen waith = a worksheet, gwaith = work becomes waith because taflen is a feminine noun. Taflenni gwaith = worksheets, gwaith does not change because taflenni is the plural noun.