[Tanialoves]

My students wonder if there is any rule for using it in the phrase would appreciate it if... All I could say was that it is a fixed phrase and should be remembered and used as it is. But can it be explained somehow?
Thanks.

[susan53]

Lexical verbs divide into two categories : transitive and non-transitive. (That's a huge simplification, but it will do for this case.)

Transitive verbs are always followed by an object - for example : I keep rabbits; John hugged Mary;We made a cake - you can't just say *I keep, *John hugged or *We made

Intransitive verbs, on the other hand, don't need an object : It rained; David laughed; Mary coughed.

Appreciate is a transitive verb - it must have an object : I appreciated his offer. You can't just say I appreciated - you must appreciate something.

Subordinate clauses starting with "if" can't act as a grammatical object, so in your sentence we need to find an "empty" object. Whenever an "empty" subject or object is needed , "it" is always used (think about It rained. It doesn't make sense to ask What rained? "It" is only in the sentence to provide the grammatical subject which the verb must have - it's "empty" of real meaning).

Or another way to think about it is to consider the sentences :
He replied immediately. I really appreciated it.
Here "it" clearly refers back to "the fact that he replied immediately". Now change that into the sentence you are asking about :
I should appreciate it if you would reply immediately.

It's really just the same thing.

[Tanialoves]

Thank you, Susan. It's a very clear explanation. And a very easy one.