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?"
Erst is used to describe a temporal order, i.e. to denote that something happens first, and something else afterwards.
A second usage pattern of erst is indeed close to only, in cases where it is used to show that so far, something has happened not very often, but this is intended to be changed.
Some more examples:
With nur one points to the state in which things are; with erst the emphasis lies on the point when things change or the progress in general.
– Ich habe nur zwei Kekse übrig. — I only have two biscuits left and their amount is not going to increase.
The second phrase is a bit strange, of course, since the Word übrig denotes that one indeed does not expect any new biscuits.
– Sie mag ihre Arbeit nur, wenn es regnet. — She only likes her work when it’s raining.
Two more examples:
– Mozart war nur drei Jahre alt, als er seine ersten Konzerte gab. — Sounds a bit contrived because one expects ageing to be a progress and therefore saying nur is strange. It would work, if he stopped ageing afterwards or if he’d always been three years old. Or if the young age were simply an additional fact but nothing which made it even more important.
– Nur auf der Zielgeraden konnte er den Läufer einholen. — Emphasises where he could catch the runner: only on the home stretch. (And they might have run 1500 m and so he had the chance a couple of times.)
Note that in the last two examples the meaning is somewhat inverted. In the Mozart example erst means that something good happened very early in the process (of aging). In the sports example it means that it happened rather late. (One could reverse both examples with the word schon/‘already’.)
– Ich war nur einmal in Berlin. — More of a simple fact. I know it’s a small number (one)
but that’s just how it is.
"Nur" means "only." Wir sind nur Erwachsende hier.
"Erst" means "only when." Erst A, dann B.
– Sie mag ihre Arbeit nur, wenn es regnet. — She only likes her work when it’s raining. – Sie mag ihre Arbeit erst, wenn es regnet. — She doesn't like her work until it's raining.