What function has the optional "ca" ("ca să tac") in this sentence really? I don't think I've seen it used like this before.
The use of "ca" here is not optional; it's wrong. This is a mistake that some native speakers make as well. You should report it.
Oops... Those inaccuracies are really annoying.
The sentence without ca is accepted as correct. 2020-03-27
I just did it without the ca and it was marked wrong. (10/26/20)