When to use bien instead of très?

Question

Here's another question about synonymous adverbs. My textbook¹ defines bien as "very, well" and très as "very". Which would you use in the examples below to convey the meaning of "very"? Are both words possible, or only one?

1. Sylvie est petite, mais Sylvain _____ grand.
2. Vous êtes _____ belles ce soir!

I am learning by self-study, so this is by no means a homework question. I just want to learn the difference between the two. Is there a rule of thumb about which to use in regard to this meaning?

¹ Easy French Step-by-Step, Rochester

Answer 1

These two words can have very close meanings. And I think you can contextually use both in each example you've provided in the question.

Here is a complete reference for "bien" (especially I.B, very close to "très") and "très".

In this sentences, you can say (or write):

Sylvie est petite, mais Sylvain très grand.

Both part of the sentence tend to be told in an equivalent way. So I think "très" is natural here.

But:

Sylvie est petite, mais Sylvain bien grand.

is not meaningless. That is to say Sylvain is really tall and more than excepted (in general, not only compared to Sylvie). Nothing bad here, except that the sentence becomes asymmetrical.

For the second one... this is matter of choice:

Vous êtes très belles ce soir!

... objectively, they are really beautiful (no more).

Vous êtes bien belles ce soir!

... beautiful too, but more than expected (or more than something that can be referenced).

... in fact, as there is "ce soir" which could tend to mean this is not always the case... so choosing a word or another without knowing what is the wanted mean is a bit difficult or subjective in my opinion.

Answer 2

"bien" is often used to mean "in some circumstances" (for example "ce soir") whereas "très" is kind of eternal truth.