Translation:Somebody is writing with a pencil.
What's the function of "er" here? Why not simply "iemand schrijft met een potlood"?
See here: https://www.duolingo.com/comment/25000664
In short, it usually sounds unnatural in Dutch to start the sentence with an indefinite pronoun (like "iemand") or a noun with an indefinite article ("een", or no article for plural).
Is there any way to distinguish between "Somebody is writing with a pencil" and "Somebody with a pencil is writing?" This sentence seems like it could mean either one, but maybe I'm just not good enough at Dutch yet to know the difference.