Can you copy and paste the section in GrammarMonster that
led you to " realise that the word 'whom' should agree with the subject of the verb".

(or give me the webpage to go to).

Let me explain the difference between who & whom with the examples below.
She is the girl ......(who/whom) I love.Here the subject is I.Verb is love but there is no object after the verb love. so in order to represent object we use whom in the blank mentioned above.
She is the girl.......(who/whom) loves me.Here the verb is loves.Object is me but there is no subject before the verb loves. so in order to represent the subject we use who in the blank mentioned above.
Use a subject after whom and a verb after who.

Thanks.
Good luck.
A sri lankan teacher.