Yes. Verb are parts of speech in which something is happening. "I
doubt that the sky is purple."
If you are confused, use these as your examples
- Knowledge= noun
- Silly= adjective
- Know= verb
So you can replace doubt in that sentence with the others.
"I knowledge that the sky is purple." Nope.
"I silly that the sky is purple." Nopers.
"I
know that the sky is purple." Yes!
The last sentence is the most accurate. We all
know that the sky isn't purple, but it fits. So you can go back and say, "I
doubt that the sky is purple."