It seems that in many situations where I might use the word *only* in English, either *nur* or *erst* is used in German.  I have the rather fuzzy idea that *erst* is used for *only* in the sense of *something that only happened once* (e.g. *He only went there to see the museum*) whereas *nur* would be used for a general rule (e.g. *She only likes working when it’s raining*).  

Even if that’s roughly right, is there more to it than that?  How about something like *I only have two biscuits left*?