Examples of errors.
So and thus are sometimes used in formal style as pro-adverbials to represent a previously mentioned adverbial of process or manner. More commonly (in) that/this way are used in this function. Consider:
Always be frank and open to your colleagues. That way/Thus/*So you'll win their trust and confidence.
Here that way represents an adverbial which, in expanded form and referring back to the preceding sentence, would be by always being frank and open to your colleagues. Thus sounds very formal, though not unacceptable, whereas so is quite odd, particularly (as here) at the beginning of a clause. For the same reason so in (1) as a substitute for by saving labor is not acceptable. Furthermore, so in this initial position is likely to be confused with so used as a result conjunct (cf. Quirk et al. 1972:§10.27) meaning 'therefore', as in
These machines save labor, (and) so/therefore/thus they seem more economical.
This would be consistent with the intended meaning in (1), but would require an independent clause--i.e. the subject of seem (machines or they) would have to appear again.
So (or thus) is more likely to appear as a pro-adverbial before adjectives, less likely at the end of a clause. Quirk et al. offer the following examples:
Professor Sands was checking the temperature in the cages where the rats were quartered. While he was so/thus engaged, he observed that one of the rats was behaving oddly.
He had expected that his luggage would be handled with due care when it was removed from the plane. However, the porters did not handle it so/thus.
Both of these examples are clearly literary or formal narratives. In informal or colloquial speech, so and (to a lesser extent) thus--particularly in clause-final position--sound odd to the point of unacceptability, as in (2), which occurred in a class discussion. In a third example given by Quirk et. al., so (or thus) would be as unacceptable as in (2):
She plays the piano with great concentration and with great energy. I'm afraid she doesn't study like that/*so/*thus.
So the author (cf. (3)) is a direct translation of so das Autor, but whereas German so can replace the entire predicate (i.e. verb + direct object clause), English so can replace only the direct object clause:
Zeit ist Geld. So das Autor.
but
Time is money. So says the author.
The English sentence requires repetition of the verb (says), and so replaces only an understood that time is money. In the German sentence so replaces an understood sagt daB Zeit Geld ist, i.e. the entire predicate, including the verb (sagt). So in English can replace an entire predicate only in combination with an auxiliary verb form, e.g.:
He said he would tell her, and he did so/so he did (i.e. he did tell her).
In combination with a lexical verb like say , so replaces a that clause, e.g.:
He knew he was right and said so (i.e. that he was right).