ておく vs てある — both mean 'already done'?

Question

I keep mixing these up. 「ドアを閉めておきました」and 「ドアが閉めてあります」both seem to describe a door that's closed now. Is there an actual difference or are they interchangeable?

Accepted Answer

ておく focuses on the preparation itself — you closed it ON PURPOSE, for later. てある focuses on the resulting STATE, and it's usually intransitive-feeling because it uses を...てある or が...てある without a clear 'doer' in the sentence. So 「閉めておいた」= I did it as prep, and 「閉めてある」= it's in the closed state (maybe you don't even care who did it).

Answer

Small addition: てある almost always takes a transitive verb + を (or が), never an intransitive one. That's actually the easiest way to spot it in a sentence.