I am playing with 2 players because no one else can, or wants to, play. One of the players feels like I target him. When he tries to do something that won't progress the game, I let him do it; if he fails, he then gets upset. I'll give you an example:
So the group was in a dungeon going against a hobgoblin captain and a group of 5 goblins. The hobgoblin makes a multiattack and rolls enough to hit and he wants to dodge the attack as a reaction. I don't know if that's possible. I looked it up; nothing in the Players Handbook, so then I say, "no you can't," because he already moved and used his action. But, he wants to look it up and he doesn't find anything; so, he is in a bad mood.
So, then the goblins try to make short bow attacks and his player is closer than 30ft and he says they can't attack because its below the minimum range. That makes no sense; if anything, it's a better chance to hit. I say whatever I don't care and keep the game moving. Then after the game I try to address it and tell him and he said that, "nope we're not gonna change it because you said it goes this way." Now I can't address him because he uses my words against me.
So, what do I do?
To all your answers: Okay so I've pretty much tried these and I'm taking a break currently letting us both take a break. Okay so I use D&D beyond and I asked some other DM and they told me what to do. So I show the player what they said and he still is defiant; I've looked up the rules showed him and he is still defiant. I'm thinking about just not playing with him but then I can't play because there would only be one player but thanks for all the possible solutions.
Best Answer
What do you do? For right now, stop playing, stop DM'ing.
Neither of you seems to understand the rules of the game well enough to play without getting into an argument.
The game looks like it is turning into a DM versus Player game; that isn't usually a sign of a healthy relationship at your game table for this particular role playing game. Bad gaming isn't better than no gaming.
But I don't want to stop playing. OK, try this ...
Get out the Basic Rules(Free download). Go to page 3, in Chapter 1, in the section on How To Play. (It's on page 6 of the Players Handbook). Sit down with this player and go over these fundamental three steps before you attempt to play again.
(Note: what the dice do is help determine success or failure of various attempts where the outcome is uncertain).
Ask this player if they accept that this is how the game is played. If they accept the fact, also, that there is no step 4:
If they don't accept that, go back to my original answer.
Learn the game together
Lastly: set up an after-the-game "how did this go?" period after each gaming session. If there were rules or rulings that this player, or the other player, or you are confused about or disagree about, then:
come to an agreement on how it will work from that point forward. The past is past.
@SPavel puts it this way:
Again, if you and your players you can't come to that agreement, you need to get another DM for this game or you need to let this game go for a while until you can agree on a more cooperative approach to the game.
If you all keep playing, take turns DMing
Since you have a small group, and you all seem to be new to the game, one way to get everyone more familiar with the game and the challenges of being a DM is to take turns being the DM. You run a few sessions, player A runs a few sessions, player B runs a few sessions. That will also help you "learn the game together" as a group.
FWIW: this answer has the elements of the action economy spelled out for ease of use. It might be worth going over those basics with your player.