Hello all,

Are all these sentences grammatically correct and do they all basically mean the same thing?

I wouldn't be surprised if this were true.
I wouldn't be surprised if this was true.
I wouldn't be surprised if this is true.

It was suggested to me that it depends on whether "this" is a thing of the past or not. For instance:

I wouldn't be surprised to learn that my sister is gay.

I wouldn't be surprised to learn that my great-great-great-grandmother was Native American.

A reference to the relevant grammar rule would be appreciated.

This is a great question. It's not the time frame of fact that matters. It's the time frame of the truth.

Neil Armstrong landed on the moon in 1969. That is true not was true. In fact, it both is and was true. So, you can make a case for either depending on what you're trying to say.

'were' is a whole different ball game. That's the subjunctive mood.

There's also a report speech angle here.

He said he was gay. (He said "I am gay".)

And, that confuses it a little more.

I think you've stumbled on something our grammar can't handle cleanly. Happy to be corrected.