How to understand the usage of "when/while doing something" in grammar?
I am watching TV, when sitting in my
I think "when" and "while" can never be used as prep.. They are conj. instead.
I remember seeing some explanation from a English grammar book, written for people speaking another language, which basicaly said "when/while doing something" is just a shorthand of "when/while somebody is doing something", where "somebody is" must appear in the main clause, otherwise this shorthand usage is not valid. For example, it is wrong to say
I watch TV, when sitting in my sofa.
because in the main clause there is no "be".
I would like to see if someone can concur or refute this explanation from rigorous English grammar point of view.