Use of “es” to anticipate a following clause

Question

I’m working my way through Hammer’s Practicing German grammar workbook (second edition) using the German grammar and usage textbook (fourth edition). I’m in chapter three and everything is going great, more than 90% correct on all exercises. Then I got to exercise eight…

Given two infinitive phrases I am to construct a main and a subordinate clause. “An anticipatory es should be used where usual, and included in brackets where it is optional.”

Unfortunately, I cannot grasp when it’s usual and when it’s optional based on the information in the textbook (section 3.6.3). For example, the first three questions have the following answers:

1. Ich habe (es) versäumt, meine Frau anzurufen.
2. Ich habe es abgelehnt, mit ihr in die Schweiz zu fahren.
3. Ich bedauere (es), dass Sie umsonst gekommen sind.

Based on the textbook, I cannot see why es is optional on 1 and 3, but mandatory on 2! Further, I really cannot see why es is not possible in the following answer to question 4:

4. Ich habe schon gewusst, dass sie fließend Spanisch spricht.

despite the book indicating to me otherwise. Maybe the answer to these questions will help me understand why answers 5 and above are as they are.

Answer 1

The es is called a Korrelat and it represents a subordinate clause in your examples (Objektsatz). In general, it's is optional.

However, a few words never take an es whatsoever.

*Ich sagte es, dass er kommen wird.
*Ich fragte es, ob er mit uns kommt.
*Wir beschlossen es, nach Hause zu fahren.

If the subordinate clause comes first, the es is always dropped. But you can replace it with das.

Mit ihr in die Schweiz zu fahren, (das) habe ich abgelehnt.

These are all rules that apply in respect to Objektsätzen.

So, you can add es to sentence 4 and drop it from sentence 2. Or can't you?

Well, there are a lot of hits on Google for "Ich habe es schon gewusst, dass" and I've heard it in spoken language, too.

Hard to find something with "Ich habe abgelehnt" (without es) that still matches your example and it indeed sounds unnatural and, though, it's fine by me. Even though I rarely drop es at all.

Perhaps it's then about idiomaticity, but I checked several sources like Canoonet and some random sites I found when searching for "Korrelat", "Objektsatz" but there were no rule or rule of thumb I found that would explain why you couldn't drop es in the second sentence.