Grammar fix

seriously doubt

not believe

To strongly think that something is not true or will not happen.

collocationB2common

Hear it in context

Notice where the phrase sounds natural.

I seriously doubt he will finish the project on time.communication
To be honest, I seriously doubt that most people recycle as much as they should, even though they know it's important.IELTS speaking

Pattern: seriously doubt + (that) clause / noun phrase

WrongI seriously doubt that he will not come.

RightI seriously doubt he'll come. (= I don't think he will)

'Doubt' already means 'don't believe' — don't add a second negative.

Patternseriously doubt + (that) clause / noun phrase

Choose the version that sounds natural in speech.

Stiff I harbour grave reservations as to whether he will appear.

Natural I seriously doubt he'll show up.

Natural; 'harbour grave reservations' is formal/legalistic.

Words that usually travel with it.

'Doubt' alone is weaker; 'seriously doubt' is much stronger and more certain.