SHOULD and OUGHT TO are both modal verbs.
In most cases SHOULD and OUGHT TO are used interchangeably today. Both SHOULD and OUGHT TO are used to express advice, obligation or duty.
However, there is a slight difference in meaning.
is used when we want to express our subjective opinion.
In other words, ‘what I think is best for you to do’.
- You should call your mother more often.
- He should apologise before she stops talking to him.
is used when we want to express an objective truth.
In other words, ‘what is necessary, and cannot be avoided’
That’s why we usually use OUGHT TO when we are talking about laws, duties and regulations.
- They ought to follow the school’s policy, or they will get expelled.
- According to this road sign, they ought to stop here.
I hope it’s clear now 🙂
P.S. Remember that OUGHT is always followed by to + infinitive.