navi posted:What do you think of:
a) It is pleasant drinking a can of beer after a long day's work.
b) It is nice sitting by the river in the evening, watching the sun set.
I am not sure about these two. Maybe when the participle clause is very long this structure becomes awkward. That's a theory I am testing, which if correct, would complement Gustavo's theory (which is obviously correct).
That's an interesting theory, but I have to say that I think both of those sentences work perfectly. As a native speaker, I do not have the slightest sense that they they are in any way off. They are 100% correct.
Again, I do have slight misgivings about "It's dangerous swimming in the river," though I hesitate to give it a question mark, much less an ungrammaticality asterisk. However, it is clearly safer to say "It's dangerous to swim in the river."
I think your "length theory" might work better if it were applied to the predicate of the expletive. I think that the following example is incorrect -- at least without a comma, on the intended reading -- because of the length of the predicate:
a1) It is just about the most pleasant thing in the world drinking a can of beer after a long day's work.
If that sentence means anything, "it" refers to something other than drinking a can of beer after a long day's work, and even then the sentence could be faulted for having a dangling participial modifier.
navi posted:PS. Sentences 5-7 would not be incorrect with a comma, would they?
Yes, that's right. They would not be incorrect (double negative) with a comma. However, they would read as Right Dislocated structures, not as structures with "it"-extraposition.