There is, of course, no uniform "catch-all" North Welsh dialect or South Welsh dialect. The variations in the Welsh language are rich, subtle, and numerous, and include specific regional dialects such as Y Wynhwyseg, or Gwentian, from south-east Wales. Similarly, major dialects have also been used in Gwynedd (Y Wyndodeg), Powys (Y Bowyseg), and Dyfed (Y Ddyfedeg), and even more localized versions exist all over, from Bangor to Caernarfon to Carmarthen.
However, there are a few aspects of the language which vary in a broad North/South regional pattern. Pronunciation differences are quite noticeable between North and South Wales, particularly the vowel sound denoted by u. In Teach Yourself Welsh, T. J. Rhys Jones notes that u in South Wales is pronounced "ee," but in North Wales the sound is produced much further back in the mouth (275). In case anyone is familiar with the International Phonetic Alphabet, I believe the appropriate symbol is a barred "i."
Another common pronunciation difference in parts of North Wales is the tendency to pronounce -au and -ai word endings as -a--trena instead of trenau. Meanwhile, in South Wales the same word endings are pronounced as -e, so afalau becomes afale in speech. You'll also hear differences in the way bod, the verb "to be," is used. For example, in the plural present tenses for first and second person, dan ni and dach chi tend to be used in the North, whereas yn ni (or dyn ni) and dych chi are used in the South. Similarly, in South Wales the past tense of bod tends to get contracted from oeddwn i to o'n i, etc.
