doubt

Clauses after the verb doubt can be introduced by whether, if or that.

• Economists doubt whether interest rates will fall in the near future.
• I doubt if she’ll come this evening.
• The directors doubt that new machinery is really necessary.

In an informal style, people sometimes use no conjunction.

• I doubt we’ll have enough money for a new car.

After negative forms of doubt, we normally use that or no conjunction.

• I don’t doubt (that) there will be more problems.

note

For no doubt meaning ‘probably’, (see here).