Modern was initially designed as a format where people could use their cards that had just rotated out of Standard. Modern's card pool includes all cards that were legal in Standard since Mirrodin and 8th Edition, as well as Modern Horizons.
These cards cannot be used in this format:
Amped Raptor · Ancient Den · Arcum's Astrolabe · Birthing Pod · Blazing Shoal · Bridge From Below · Chrome Mox · Cloudpost · Dark Depths · Deathrite Shaman · Dig Through Time · Dread Return · Eye Of Ugin · Field Of The Dead · Fury · Gitaxian Probe · Glimpse Of Nature · Golgari Grave-troll · Great Furnace · Grief · Hogaak, Arisen Necropolis · Hypergenesis · Jegantha, The Wellspring · Krark-clan Ironworks · Lurrus Of The Dream-den · Mental Misstep · Mycosynth Lattice · Mystic Sanctuary · Nadu, Winged Wisdom · Oko, Thief Of Crowns · Once Upon A Time · Ponder · Punishing Fire · Rite Of Flame · Seat Of The Synod · Second Sunrise · Seething Song · Sensei's Divining Top · Simian Spirit Guide · Skullclamp · Summer Bloom · The One Ring · Tibalt's Trickery · Treasure Cruise · Tree Of Tales · Umezawa's Jitte · Up The Beanstalk · Uro, Titan Of Nature's Wrath · Vault Of Whispers · Violent Outburst · Yorion, Sky Nomad