Ought to has a similar meaning to should. They both express the conditional: in other words, they refer to a recommended future action.
You ought to pay him back shortly. You should pay him back in the near future.
She oughtn't speak so loudly. It would be better if she didn't speak so loudly.
You ought not (to) speak with your mouth full. You should not speak with your mouth full.
Used less frequently than should, ought is always followed by a verb in the infinitive with to, except in the negative form, where we don't need to add to:
You ought not (to) ask so many questions. It would be better if you didn't ask so many questions.
Note: ought to does not have a past form. It is only used with reference to the present and the future. Ought to have + past participle is used to express (past) regret:
I ought to have kissed him when I had the chance. I regret not kissing him when I had a chance to.