Any differences between 「学生の時」 and 「学生だった時」?

Question

They seem to mean the same, although I hear 「学生の時」 much more often.

Another example I'm unsure of is: 「犬が病気(の・だった)時、病院に連れていきました。」 "I took the dog to a vet when it was sick." Are both options OK?

Answer 1

Both can be used almost interchangeably, but I guess の時 is more often used as you feel.

One obvious limitation is that だった時 cannot be followed by a generic statement.

• 犬が病気の時はこの病院に連れてくればよい
• ｘ犬が病気だった時はこの病院に連れてくればよい

Same for repeated event:

• 犬が病気の時はよくこの病院に連れてきた
• ｘ? 犬が病気だった時はよくこの病院に連れてきた

だった時 fits okay in one-off events like 病院に連れて行きました so that 犬が病気(の・だった)時、病院に連れていきました are both fine. (Still I feel の is more common, though.)