I was under the impression that I use the word 'Was' when I am referring to something in the singular, ex) he was in the shopping mall last night.

I was also under the impression that the word 'Were' is used only in the plural sense, ex) the students were baking cookies.

However, a recent question on mgmat is as follows:

A recent and popular self-help book wryly notes that if adolescence was not so painful, it would have a droll comedic aspect, at least in retrospect.

The correct answer choice is : were not so painful, it.

Can someone please help me with this rule? I would have picked that answer prior to studying SC books, but now I am completely confused.

Please help,

Vinnie