^kani ra akong ma-share on punctuation, bro:
I loved him, but he didn't love me.
I loved him but wasn't able to express that love well.
In the first example, we can make two sentences if we take out but.
I loved him. - OK
he didn't love me. - OK
~use a comma
In the second example, we cannot make two sentences.
I loved him. - OK
wasn't able to express that love well. - Not OK; there is no subject
~do not use a comma