doubt
Doubt can be a noun or a verb.
‘doubt’ used as a noun
If you have a doubt or doubts about something, you feel uncertain about it and you don't know if it is true or possible.
I had moments of doubt.
The report raises doubts about current methods.
‘no doubt’
If you have no doubts about something, you are certain that it is true.
Francesca had no doubts about the outcome of the trial.
If you say there is no doubt that something is true, you mean that it is certainly true.
There's no doubt that it's going to be difficult.
You must use a that-clause after there is no doubt. Don't use an if-clause or a whether-clause.
You add no doubt to a statement to say that you are assuming that something is true, although you can't really be certain about it.
As Jennifer has no doubt told you, we are leaving tomorrow.
The contract for this will no doubt be widely advertised.
*
‘doubt’ used as a verb
If you doubt whether something is true or possible, you think it is probably not true or possible.
I doubt whether it would work.
I doubt if Alan will meet her.
If someone says that something is true, or asks you if something is true, you can show that you think it is unlikely by saying I doubt it.
‘Do your family know you’re here?' – ‘I doubt it.’
Be careful
Don't say ‘I doubt so’.
Don't say ‘I doubt so’.