Question

I’ve noticed that only when I scold him _______ he take me seriously. *

Answer 1

I have noticed that only when i scold him ,only then he takes me seriously.

This is the human nature.

Since we are kids,we have taken some one serious only after getting some sort of scoldings from them.

Be it a kid or an adult sometimes scolding them becomes necessary for their dear ones.

Answer 2

Answer:  

I’ve noticed that only when I scold him must he take me seriously.

Explanation:

In the above referred sentence, the speaker is talking about something which is the fact and a compulsion or to show necessity i.e. in the blank, one can use should or must. ‘Must’ is used in a sentence to show the compulsion of anything while ‘should’ has the same use but ‘must’ is much stronger than the word ‘should’. It is used in severe situation while ‘should’ or ‘ought’ are the words which are milder than ‘must’.