past?

I think are is acceptable as you state a general fact.

Previously, I post the following question.
"In the West, it's common to invite friends to their house/houses."

The answer I got then was "houses." I am wondering if there are any
differences between the question I just asked and the one I asked
several days ago.

I always have great difficulty teaching how to choose "plurals" and "singulars."
I really appreciate

Sorry, I meant to post this.

Me, too. I'd choose the present tense.
But the book I use when I teach says, "were."

Maybe both are acceptable, but it's very hard to
convince my students, since the book says otherwise.

I'd like to know whether there are any grammatical,
I mean rules which we are supposed to follow, especiallly
in written English.

As I non native I am learning from the native speakers to use the form which appears more meaningful in the context instead of blinding following the book.

When students disagree I show the discussion threads here to convince them.

I think we agree on this.

And as for rules at some place I ask them follow the rules rather the convention. Indirect speech is one of those

teacherJapanWhen I got a camera at the age of ten, I realized how interestingvisual images are/were.

At the time, I thought "Visual images are very interesting£.

When I later report this, I can backshift 'are' to 'were', to place my thought in the past. However, if I still think the same at the the time of reporting, then I can use 'are' to present my