That's quite a broad question, but I guess you're referring to some of its "special" uses, such as in describing physical or mental states (illnesses, emotions, etc. e.g. Mae annwyd arna i = I have a cold; Mae ofn arna i = I'm afraid), or when it is used with a verbal noun to mean "about to" (e.g. Dw ar fynd = I'm about to go). Let me know if you have a more specific issue in mind and I'll try and explain it to you better.