[Grammar] verb + it + that- if- or when-clause: When to use?

[John Laudenberger]

I have been studying the Cambridge Advanced Grammar in Use and found some information about the use of "it" after some verbs followed directly by a that-, if- or when- clause in Unit 97 (page 194). I would like to know why after the verbs "dislike, enjoy, prefer and understand" we have to use "it."
For example: "We always enjoy it when they stay with us" (and not We always enjoy when they....)
What do these verbs have in common?

And why, after "argue, discover, emphasize, notice, predict, remember" we cannot use "it"?
Example: I can't remember when I last saw her. (not .... remember it when).

I would also like to know if there are any books which contain such information.

Thank you.

 

[emsr2d2]

I have been studying the Cambridge Advanced Grammar in Use and found some information about the use of "it" after some verbs followed directly by a that-, if- or when- clause in Unit 97 (page 194). I would like to know why after the verbs "dislike, enjoy, prefer and understand" we have to use "it."
For example: "We always enjoy it when they stay with us" (and not We always enjoy when they....)
What do these verbs have in common?

And why, after "argue, discover, emphasize, notice, predict, remember" we cannot use "it"?
Example: I can't remember when I last saw her. (not .... remember it when).

I would also like to know if there are any books which contain such information.

Thank you.

"We always enjoy it when they stay with us". In that sentence, "it" refers to "the time that we spend together" or something similar.
"I can't remember when I last saw her". There is nowhere that you could put "it" in that sentence as there is nothing to replace. If anything "it" would mean "when I last saw her". As that already appears in the sentence, there would be no reason to duplicate it.