Its best to keep the rhizome of the Anubias plant above the substrate-otherwise it can rot and die. Anchor it to something like driftwood with a rubber band or cotton thread. It will attach itself to real driftwood and send its roots into the substrate on its own. Or, you can place something on the roots to hold the plant down as long as you don't bury the rhizome.
Sword plants-need to be planted in the substrate, however, its crown needs to be slightly above the substrate line so it doesn't rot. It is better to plant it too shallow than too deep.