vor X Jahren = point in time = X years ago seit X Jahren = beginning of a span in time that continues to the present (or the narrative time) = since X years ago X Jahre lang = length of period, neutral = for X years Perfekt = indicates that the action is completed Imperfekt = indicates that the action was ongoing (at that time) and not completed Präsens = indicates that the action is still ongoing. When it's not important of the action is completed or not, Perfekt and Imperfekt can both be used interchangeably. Ich habe hier vor fünf Jahren gewohnt = I've been living here five years ago, but now I live somewhere else Ich habe hier seit fünf Jahren gewohnt = I've been living here for five years, but I've just moved somewhere else Ich habe hier fünf Jahre lang gewohnt = I lived here for five years, not necessarily up to now Ich lebe hier seit fünf Jahren = I've been living here for five years (and I still live here) Ich lebe hier vor fünf Jahren = makes no sense Ich lebe hier (schon) fünf Jahre lange = I've already been living here for five years Ich lebte hier seit fünf Jahren = I had been living here for five years, from the point of view when the action takes place Ich lebte hier vor fünf Jahren = Five years ago, I was living here for some longer (unspecified) time. Ich lebte hier fünf Jahre lang = I was living here for five years The tenses are used differently in English and German (and I hope I got the English tenses right, they are confusing for me, too :-)

Both sentences are wrong. Ich WOHNE hier seit fünf Jahren. Ich studiere seit zwei Jahren. Seit zwei Stunden ist er bewusstlos. You never use VOR in this expression. vor = before; ago

I believe the first example would be " Ich hatte" since you used to live there 4 years AGO. The second one is SINCE.