steam problems

so here is my story khv please help me.
my father got back into pc gaming and i thought
it would be a great opportunity for my hacking
i am interested in doing hl2 and portal mods
but he has hl2(half life 2) installed under his user name
and he plays it all
the time(except when he is at work) so i can't just
mod the maps etc and have him not notice
and not saving is not an option for me
so i wanted to know how can i put half life 2 under my account
and him still have his with ours having nothing to do with each other
so there would be no problem with me playing while he is at work

sorry for the long speech

• yes i am new to PC gaming
• yes he has the hard copy of Half life 2
• yes he bought it and everything is all legal
• yes i already have a steam account

please help me guys!!

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