Quote:
1. Frank is a fine athlete, as was his father before him.
2. As his father before him, Frank is a fine athlete.
I think in the first case the word 'as' is an conjunction. . . .
Good try, but take another look at where 'as' sits in the sentence.
1. Frank is a fine athlete, as was his father before him.
'as' is sitting in a subject position, which makes it a nominal in function, notably a relative pronoun - so also says my Oxford. Here's the example they give, She is an actor, as is her husband, to which I will add, an actor also is her husband. Notice the verb 'is'. If 'as' were a conjunction, that would make 'is her husband' a subjectless clause. (Conjunctions cannot function as subjects.)
Getting back to your example sentence 1., "as" modifies the noun phrase "a fine athlete":
Frank is a fine athlete, as was his father before him.
Frank is a fine athlete; a fine athlete also was his father before him.
'as' functions as the subject of 'was' and stands for 'a fine athlete'.
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The second sentence, on the other hand, uses 'as' as [a] preposition.
In that context, "As" is synonymous with "Just like", a conjunction:
As his father [was a fine athlete] before him, Frank is a fine athlete.
Just like his father [was a fine athlete] before him, Frank is a fine athlete.
Frank is a fine athlete, just like his father was [a fine athlete].
There's your conjunction. :-D SVSC as/just like SVSC
Frank is a fine athlete, just like his father was a fine athlete.
In short, my understanding is this: 1. houses a relative pronoun, whereas 2. houses a conjunction.
1. Frank is a fine athlete, as/which was his father before him.
2. As/just like his father before him, Frank is a fine athlete.
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And I also suspect that the second sentence is very likely to be followed by descriptions of Frank rather than his father.
Good eye! It has to do with (a) the position and (b) function of the pronoun "his". That pronoun (a) sits in a topicalized clause and (b) refers to Frank.
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The following two are shown together in the examples for [in the same way] in Collins Cobuild.
I will behave toward them as I would like to be treated.
The book was banned in the US, as were two subsequent books.
It really confuses me.
'as' functions as a conjunction in the first example. The structure tells us that. Punctuation is also a good test:
Conjunction
Do as I do. (no comma; 'as' functions as a conjunction)
Relative Pronoun
They won, as you know. (comma; 'as' modifies 'They won'.)
As for Collins Cobuild's second example, 'as' modifies 'banned':
The book was banned, as were two subsequent books.
The book was banned; banned also were two subseqent books.
I may be right; I may be wrong. Please feel free to argue your case. 8-)