Sheffield is a city in the county of Yorkshire in Northern England. Every region of Britain has its own pronunciation, which may differ a lot or a little from the standard, neutral or RP (Received Pronunciation) accent which you are probably used to. Accents in London are different from accents in Sheffield or Birmingham or Liverpool - so you can't say that any one place has 'more' accent than anywhere else. The pronunciation is simply different in different places. It also depends who you are talking to. Accents vary according to age, class and education. You may well find that a teacher from Sheffield is easier to understand than a window-cleaner from London. I'm not sure where you got the den/then example from. It isn't a particular feature of Yorkshire English. However, one of the most noticeable features of the Yorkshire accent, and Northern accents in general, is the pronunciation of the 'a' and the 'u' sounds. In Northern accents, the word 'ant' (the little insect) is pronounced the same way as 'aunt' (your female relative), and the word 'luck' might be pronounced the same way as 'look'. But this isn't a problem - people from all over Britain can always understand each other, and if you were to go to Sheffield it would take you no time at all to get used to how people speak there.