Tense is something I often have problems with. For example, if you write in the past tense about something which is still the case.
If I'm writing a brief review - bad memory - of a book I've just read, should I say:
"While I read this book I became totally absorbed by the sense of place which (was or is? The book still exists.) depicted with small telling details by the author."
This is a bad example; but I can't think of anything better right now.