I would appreciate it if … / I would be grateful if …
Different formsAppreciate is a verb and normally needs an object in this construction: appreciate it if. Grateful is an adjective after be, so would be grateful if is complete without it.
I would appreciate it if …
Use when
You make a polite request using appreciate as a verb; it supplies the object before the if-clause.
Example
“I would appreciate it if you could reply by Friday.”
I would be grateful if …
Use when
You make a polite request using grateful as an adjective after be; no object such as it is required before the if-clause.
Example
“I would be grateful if you could reply by Friday.”
Quick check
I would appreciate if, I will appreciate if, I would be grateful it if, I would grateful if
Usage note
Would is the conventional polite form. The main grammar problem with 'I appreciate if' or 'I will appreciate if' is the missing object after appreciate.