As soon as we are finished, we'll let you know.
Is this correct too?
Yes :up: - in fact it accounts for the odd-looking participle. A user of English as a second language has misheard 'As soon as we're finished...'. Of course, there are people who would argue that as this mistake is common, and many competent speakers have learnt 'as soon as we finished' as meaningful (not just acceptable, but correct), it is therefore
grammatical. They're using the term 'grammar' with what Chomsky called 'systematic ambiguity', in a way that seems to me unhelpful to language students (except in that 'Making mistakes is OK' is an attitude that makes life easier... if that's what you want).
b