In doubles you've gotten comfortable sending two Pokémon out at once, using Fake Out to get a free hit, and hiding your sweeper behind a partner. Singles strips all of that away. One Pokémon on each side — you bring a team and pick a subset to use (check in-game for the exact numbers), then take turns attacking until one side is out.
With no partner, switching becomes your only form of cover. When you're in a bad spot in doubles, your teammate can take the hit. In singles, you pivot out to a better matchup — meaning the Pokémon whose typing and stats let it go toe-to-toe with whatever the opponent has out. A lot of games are decided before you even see a move, purely by who reads the team preview better.
Prediction matters more here. In doubles you're tracking four Pokémon at once; in singles you're only watching two, but your opponent can read you just as cleanly. The central loop of mid-game singles is: do I attack, do I switch, or do I click the move I think will hit them if they switch in? Get that wrong and you lose momentum fast — no partner to bail you out.
Entry hazards (rocks or spikes that deal damage to incoming Pokémon when they switch in) are much stronger in singles than doubles, for a simple reason: doubles teams don't switch nearly as often, so hazards don't get many chances to proc. Singles teams switch constantly. Stealth Rock in particular shows up on most teams in any singles format (subject to in-game availability).
Setup sweepers — Pokémon that spend a turn buffing with moves like Swords Dance or Calm Mind before going on the attack — are more viable too. In doubles, the opponent ignores your sweeper and attacks your partner, or hits you with Fake Out to interrupt. In singles, your opponent's only options are to attack you or switch away, which often gives you the time to complete a setup.
Status conditions like sleep and paralysis hit harder here too. In doubles, one paralyzed Pokémon means the other can carry the turn. In singles, one status condition on your Pokémon is one turn you might not get back. Champions has adjustments to how status works, so check the in-game details — but the point stands.
Incineroar is the clearest example of a Pokémon that doesn't translate. In doubles, Intimidate drops both opponents' Attack at once, Fake Out gets you a free flinch, and it functions as glue that holds the whole team together. In singles, most of its support toolkit is dead weight, Intimidate only drops one Pokémon's Attack, and its damage output isn't good enough to carry. Flip that: an offensive Pokémon that looked too slow or too self-sufficient for doubles might absolutely shred through a singles ladder.
Mega Evolution still works — one activation per battle, same as in doubles. The decision about when to Mega is roughly the same: you're getting a big stat and ability upgrade, and you want to time it so it either wins you the current exchange or forces a switch the opponent doesn't want to make (check in-game for specifics).
What carries over from your doubles experience: reading a team preview for win conditions, understanding that speed determines who goes first, and knowing which abilities and items are universally useful.
What changes most is your mental framing. In doubles you ask "how do these two Pokémon work together?" In singles you ask "who has the better matchup on field right now, and is it better to press or to switch?"
Your turn
Browse the dex and pick three Pokémon you've used in doubles. For each one, ask yourself: does it win a 1v1 on its own damage and speed, or did it only work because of the support around it?