#2 is correct.
Please let me know which one is correct.
1. I doubt if it is correct.
2. I doubt it is correct.
#2 is correct.
So is #1
(1) According to Mr. Michael Swan's very popular Pratical English
Usage, a clause after "doubt" may be introduced by:
(2) The Columbia Guide to Standard American English agrees.
(3) Mr. Bryan A. Garner's A Dictionary of Modern American
Usage (a book that many people trust) says:
doubt that (use in negative sentences, questions, and statements
of skepticism [you think it may not be true]).
doubt whether (use mostly in affirmative statements [sentences that
are not negative]). Again with skepticism.
doubt if (suggests a conditional sentence). Mr. Garner says "doubt if" is
"less sound" in American English. In other words, he does not seem to like it.
Mr. Wilson Follett in his Modern American Usage says:
I doubt that I can go. = I do not think that I can go. (Negative)
I doubt whether I can go. = Not certain. Just a little negative.
I doubt if I can go. = same as "doubt that."
The New Fowler's Modern English Usage (Third Edition) tells us that
since the last part of the 19th century, "doubt that" or no conjunction
is often used with affirmative sentences. Its example: "I doubt that the
White House is responsible for ...." Its example with no conjunction:
He doubted [Mr. X] would sue him.
Tom: I have spent 24 hours doing this math problem. I have the
Martha: Sorry, but I know that you do not understand math
very well. Excuse me for saying this, but I doubt that it is
correct./ doubt it is correct.
(I would not recommend "if" or "whether.")
***** NOT A TEACHER *****
I have read the preceding posts with interest, referred to other authorities, and tried to analyse how I use if, whether and that with doubt. The only conclusion I could reach was that some speakers use the words in one way, and some in another. I, for example, could use if or whether in TheParser's last example. In that specific example I believe that I would actually use at, and I feel that there would be a very slight difference if I used either of the other two, but I know that I cannot find 'rules' that would satisfy all native speakers.