Ought is used to say what would be right or wise to do. You ought to go, ought to have known, ought not to.
It is the only modal verb in English that takes to before the following verb — you ought to go, but you should go — which is a real oddity. Ought to have done expresses regret about the past. The negative is ought not, and oughtn't is rare and awkward. Note that ought is the old past tense of owe, so what you ought to do is what you owe.
1verboughtUsed to say what would be right or wise to do.
“You ought to see a doctor about that cough.”
ought is a modal verb.
Old English ahte is the past tense of agan, to own or owe. Ought and owe are the same verb, which is why obligation and debt are the same idea here.
Proto-Indo-European
*h₂eyḱ-
Old English
āhte
Middle English
oughte
English, today
ought
From Middle English oughte, aughte, aȝte, ahte, from Old English āhte, first and third person singular past tense of Old English āgan (“to own, possess”), equivalent to owe + t. Cognate with Sanskrit ईश्वर (“capable of, liable”).
Etymology from Wiktionary, CC BY-SA