"Is the food tasty?"
Translation:Ydy'r bwyd yn flasus?
This is explained in the course notes, but you must use the web-based version of Duo to see them. It is worth going through them before you start each new section - we have written them to address the common questions that people ask about each topic.
A summary of the uses of yn and their different mutations is here
Not in this case - Ydy...? is the form used at the start of sentence for a question in the modern colloquial language.
You can use yw as part of other questions, though. For example:
- Pwy yw/ydy e? - Who is he?
- Beth yw/ydy hwnna? - What is that?
- Sut un yw/ydy Dewi? - What is Dewi like?