My senior English teacher was a tad bit confused where the subject for was is in this sentence:
As was traditional for unmarried women, Jane lived at home her entire life.
Strange as it may sound, the subject of "was" is, in the opinion of many renowned grammarians (please read N.B. below), the relative pronoun "as". In that sentence, "as" is not a conjunction but a relative word equivalent to sentential relative "which", but, unlike the latter, which always appears after the sentence to which it refers, "as" can precede the sentence, as a fronted non-restrictive relative clause:
N.B. Here is the evidence that accounts for my answer (sorry for the delay in finding it). Please note that I'm not saying that the answer I am giving is the only one possible -- quite on the contrary, I admitted in the comments below that there may be different views on this, as well as on other grammatical matters.
On page 126 of "Studies in English Language and Teaching: In Honor of Flor Aarts," under 2.2.1 As introducing finite clauses, the author starts admitting how much controversy this question arouses. You can read the section in question here. Then, the author mentions that Curme (1931:219) lists as with the relative pronouns and, later on, that Quirk et al (1985:1116) also claim that 'as may function as a relative pronoun' in sentential relative clauses. Even Fowler refers to as as a relative pronoun in the First Classic Edition of his Dictionary of Modern English Usage (see here). (So far as for the refusal to recognize "as" as a relative pronoun.)
Now, Fowler goes even further when he claims here that 'failure to recognize that as is a relative pronoun sometimes produces mistakes...' as *?it (note the asterisk/question mark before "it") would be the case if we wrote "it" before "would be" in this last clause, or before "was" in the clause at issue (the result in the latter case would not be ungrammatical or dubious, but the clause would change its meaning to reason): "As it was traditional for unmarried women (to do so), Jane lived at home here entire life."
Final note: I do hope that those who cast a downvote take it back kindly disregarding my initial apparently unfounded opinion (based on literature I claimed to have read but did not have available at the time) and considering, in the face of the evidence provided, the opinion of the well-reputed grammarians cited. Thank you.
As __ was traditional for unmarried women, Jane lived at home her entire life.
It has no overt subject.
The expression in bold is an adjunct of comparison with the preposition "as" as head. The comparative clause functioning as complement to "as" is structurally incomplete as I've marked with __ to represent the missing subject, though it is recoverable from the matrix: what was traditional for unmarried women in those days was living at home.
The subject of "was" is apparently missing. This is a complex sentence, so not all parts have to have all the elements of a main clause. The main clause of the sentence is the second part, "Jane lived at home her entire life."
I found the following explanation in"A Short Overview of English Syntax based on The Cambridge Grammar of the English Language," by Rodney Huddleston:
Comparative clauses generally function as Complement to the prepositions as and than:
- i a. I'm as ready as I ever will be. b. As was expected, Sue won easily.
- ii a. More people came than I'd expected. b. He has more vices than he has virtues.
The distinctive property of such clauses is that they are structurally incomplete relative to main clauses: there are elements understood but not overtly expressed. In [ia] and [iia] there's a missing Complement and in [ib] a missing Subject. Even in [iib] there's a missing Dependent in the Object noun phrase, for the comparison is between how many vices he has and how many virtues he has. The fact that there's some kind of understood quantifier here is reflected in the fact that we can't insert an overt one: *He has more vices than he has ten virtues.
(Some formatting adjustments made to emphasize the relevant example and to make the formatting work on this site)
Gustavson's answer shows the controversy that there has been about the proper classification of "as" in this construction (conjunction, preposition or even relative pronoun). I don't know enough about syntax to explain the arguments for and against each position. The conjunction/preposition uncertainty is apparently based on the fact that traditionally, it seems all words that introduce a clause were considered to be conjunctions, but some current analyses (such as the one that Huddleston presents) consider it possible for prepositions to take clauses as a complement. There is a relevant footnote in the overview that I will quote:
 In traditional grammar it is not she left but before she left that is analysed as a clause, with before being here a subordinating conjunction rather than a preposition. We present arguments in favour of our analysis on pp. 1011-14 and 129-30 respectively of [The Cambridge Grammar of the English Language and A Student's Introduction to English Grammar].
As certainly seems to function as a preposition in some commonly used constructions, such as "We're as good as them!" with objective "them" rather than subjective "they".