I am just beginning to formally learn Mandarin but have spent a lot of time in China and do have a rudimentary grasp of the language. One thing I cannot find in any lessons so far is an expression I hear very frequently. Phonetically it is "Ni ge" and, in conversational context, appears to me to be a "filler" as it is always used at the end of a sentence and often repeated several time. Similar to saying "you know" at the end of a sentence in English when you are trying to grasp for a correct word. Can someone help?

Hi - it is a filler word - like the english "uh" or "um".

Hi there: I know what you mean. The right phonetics for that expression is "nèige" ("那个"in characters). We've actually covered that expression in the script. You didn't realise that is the expression you hear every day in China because its standard pronunciation is "nàge" (as shown in our lessons), whereas in everyday life, people tend to pronounce it into "nèige", especially in some reasons. As you have understood, people around you use it a lot at the end of their sentences. Actuallly in that case, that expression doesn't really mean anything. It is just that people need some time to put their mind together and think about what they wanted say next. So they use that expression in between. I would say, it functions more like "Um, Umh" in English. Hope that helps you understand it better.