doubt

[jiang]

Dear teachers,

I was taught that doubt should be followed by 'whether/if' clause in declarative sentences and it should be followed by 'that' clause if it is an interrogative or negative sentence. But I found the following two sentences in Cambridge dictionary:
1. I doubt whether/if I can finish the work on time. (This is correc according to the grammatical rule).
2. [+ that] They had begun to doubt that it could be completed in time.
This is a declarative sentence but it is followed by 'that' clause. Could you please explain why?
Looking forward to hearing from you.
Thank you in advance.
Jiang

[baqarah131]

To me, "doubt if" is slang, and "doubt whether" is very formal.

I doubt if you can do it. (Slang, but very common)
I doubt whether you do do it. (Formal, not common)
I doubt that you can do it, or
I doubt you can do it. (Normal conversation.)

I don't like the rule you were taught. It doesn't reflect very well how we actually use "doubt."

regards
baqarah

[jiang]

Dear baqarah,
Thank you very much for your explanation. Now I see.

Best wishes,

Jiang

Originally Posted by baqarah131

To me, "doubt if" is slang, and "doubt whether" is very formal.

I doubt if you can do it. (Slang, but very common)
I doubt whether you do do it. (Formal, not common)
I doubt that you can do it, or
I doubt you can do it. (Normal conversation.)

I don't like the rule you were taught. It doesn't reflect very well how we actually use "doubt."

regards
baqarah