This is just notes about the standard 5.0 rules for bonus actions and also two weapon fighting, both of which came up in the course of play last night.
The base rules for bonus actions are on page 189 of the PHB.



This phrasing, "allows (you) to take a bonus action" is chosen because there's no ability to take a bonus action unless something grants you that. The rules then immediately hit us with:



This is probably part of their "lets make 5e simple" idea- that you aren't supposed to view your character as having a Standard Action button, a Reaction button, and a Bonus action button, and build so that you can press al three of them each round. But of course, that's exactly what you should do. But their design intention wasn't that. Like so many good intentions from 2012, this didn't turn out as expected.



This line makes it clear- the bonus action is a resource you should use each round, and if you can't, go eventually find a magic item that gives you one or something. It also is worth reiterating that you only get one per round. I don't think any effect ever gives you a second one. In Baldur's Gate 3, there's at least one subclass and maybe an item that work by giving a second bonus action, and in the post BG3 world it's worth noting that this is only true under Larian's houserules.



The first half of this I pointed out when it didn't matter, but at some point it might. Pretend you are a dual wielding fighter 7 and therefore have extra attack. You can attack with your left handed weapon, then move, then use your bonus action to attack with your right handed weapon (not your left, you haven't fulfilled the conditions to get that this round), then move again, then attack with either weapon using your extra attack feature. This uses up both your "Action" (you used the attack action) and your "Bonus Action" for the round (the game feature "Two Weapon Fighting" provided you with an available bonus action, which you used).

Finally here's the "Two Weapon Fighting" rule from PHB 195:



So this is a game feature that gives you a bonus action, and the conditions for it to give you access to a bonus action are, you are holding two light melee weapons and you attacked with one of them using the "Attack action". Normally you have to use your action for your turn to do an "Attack action", but sometimes special or lesser effects give you one, such as the haste spell. It also halves the damage (mostly) of the attack by having you not add your ability bonus to the offhand attack. Of course, many interested dual wielders will have abilities that cancel this- the fighter, for instance, can eliminate this penalty with the "fighting style" feature, which has an option named:
Two-Weapon Fighting. When you engage in two-weapon fighting, you can add your ability modifier to the damage of the second attack.
Rangers get this exact ability option (it doesn't stack because abilities with the same name don't stack), as do college of swords bards. Additionally, anyone can get it via the Fighting Initiate feat.

For my games, the ninja class feature "weapon focus" has an option named "Dual Weapon Focus" which provides this same buff (and a different buff at 5th level when this one stops mattering). It doesn't stack with "fighting style" because "weapon focus" says it does not. Eleventh level rogues, meanwhile, now just get the "fighting style" feature with only two-weapon fighting as an option (if you have it from some other source, you can go pick something else from that same source so that you end up with two fighting styles).

Ok, that's it for the rules notes. Now for the airing of grievances.

I consider the baseline asymmetry of dual weapon fighting distressing and confusing, especially because the game immediately expects you to expend a build resource to add it right back- unless you are a rogue. Then the game considers it reward enough to use your bonus action to have a second chance at sneak attack if the first one failed. (In my house rule document I eventually give all rogues this relatively minor thing).

I don't think it's great design to be like "no one has bonus actions unless some feature gives it to you BUT there's a global rule we tucked between opportunity attacks and grappling that can give any creature in the world a bonus action as both weapons are LIGHT but we won't reference this under the LIGHT property AND you gotta change the damage formula to NOT add your attribute BUT actually you're probably gonna take a feature that adds it right back DOWN SET HIKE"

It doesn't annoy me enough to try to change it though. I was really hoping 5.5 would fix this, but like, oh it gets so much more twisted.
In 5.5, the dual wielder feat lets you attack with a bonus action (which you can already do), and the nick weapon mastery lets you move the attack from the normal two weapon fighting bonus action to the attack action so if you have both you can attack twice with the offhand, once by moving the normal one from bonus action to attack action (using nick) and the second time because the dual wielder feat is a separate bonus action from the normal one so you have to pick one and of course you pick the one that the nick property didn't override. Like is there truly no other way to write this shit for real.