Ffxiv has a lot of content available in the free trial but it is not really a “free mmorpg” since if you want to do content that released less than 6 years ago, or join/use FC guilds or linkshell chats, or trade, have any significant quantity of gil, get housing, or use player markets, then you have to buy the game and pay a subscription. Mahjong is free though and it’s quite a good mahjong client.
That said, it is a good way to figure out if you like the basic gameplay systems: the UI and menus which are very customizable, controls (switch to legacy if using M&KB), tab target combat and global cooldown, main scenario questing, instanced dungeons and normal mode bosses, unlocking multiple jobs to try out.
All this with the caveat that basically all jobs at level 70 feel like an empty shell of the combat rotation of level 100, and high end content is only available and populated for the most recent patches + the 6 ultimate raids.
(cont.) Guild wars 2 is another mmo that is worth trying to see if you like the feel of the action combat. You can again play a whole lot free, but you do have to pay if you want the expansion content (which i feel is kind of diabolical to lock mounts behind max level expansion content). I also do not like that you can buy in game currency with real money, and then buy legendary (best in slot) weapons off the trading post with in game currency.
Theres maplestory which is mainly singleplayer, just make sure you make your char on the reboot (non-p2w) servers. Scratches the itch.
Honorable mention to private/classic servers like tera classic which i’ve been meaning to try out. The population being tiny is the most obvious factor but it is nice that the game has been preserved at all for people to play.