I am really confused about this. My teacher tells me that after "in case" I always have to use subjunctive and this is the only acceptable form. For example : "Take a coat in case it should get chilly tonight." instead of "Take a coat in case it gets chilly tonight." My problem is that I can't find this rule in any of the grammar books I have and what is more my coursebook's answer key never uses subjunctive after in case. My hunch is that using subjunctive after in case is old-fashioned, but I am not really sure given that it doesn't appear anywhere. Please help me understand this, thank you!
2 Answers
The subjective is optional and not required after "in case". It is correct to say "Take a coat in case it gets cold."
As noted in comments, "should" is a modal and not the subjuctive. The subjuctive would be "Take a coat in case it get cold." (Which most speakers would find strange.)
[1] Take a coat in case it should get chilly tonight.
[2] Take a coat in case it gets chilly tonight.
I think your teacher is confused. The idiomatic "in case" does not license subjunctive clauses (we can't say *"Take a coat in case it get chilly tonight"), and in any case the presence of "should" rules out any possibility of the clause being a subjunctive.
In both examples, the preposition phrase "in case" is introducing a declarative content clause functioning as an adjunct of implicit purpose, where the meaning is Take a coat in order to avoid getting cold if it gets chilly tonight.
The only difference between the two is that [1] infers that it becoming chilly tonight is less likely than in [2].