If your players have been doing this for a year and are still having difficulty remembering something they have done hundreds of times, I suspect that one of a few things is happening:

• Do your players have their full character sheets filled out? Have they written in the calculated bonus for each attack they make? Have they added up all the bonus points in each skill?


• Have you encouraged your players to remember on their own or to coach each other? Or do your players reflexively ask you for help when they don't actually need it? Have they just become used to being reliant on you for those things?


• Have you recently given an overview lesson on how that general class of rolls works? Is it possible that they think that each roll is mysteriously unique?

If you answered yes to all of those and it's still an issue, then I would talk with the players individually and try to figure out where their confusion is. Make sure to not come off as overly concerned- it is a small thing, after all- but instead keep it focused on making sure they are able to have a good time. You could say something like "Hey, I noticed that you still seem a little confused about making attack rolls. It's fine to ask for help, but I was wondering if you could tell me about why it's confusing so that I can be more effective."

I hope that helps- Good luck!

Cheat Sheets

When I play slightly more complex characters I make a "Combat Turn Cheat Sheet". The idea is I made this character work really well one way and I never want to forget a step so I start with the basics.

Attack Roll: 1d20 + Str + BAB + 1 (magic)

Damage: 2d6 +1d4 (GWM) + Str + 1 (magic)

additional steps or alterations just keep going with them looking like step on a stair case (if/else statements for programmers). Some things I would add are poisons if used often, feats or special abilities that tack on extra rolls.

For example, this is what my 3.5e Crusader looked like.

• pick a maneuver




• Attack Roll: 1d20 + Str (8) + BAB (8/3) + 3



• 
  
  • Damage: 3d6 + Str (8 * 1.5) + 3
  
  


• 
  
  • Maneuver: apply maneuver
  
  


• 
  
  • 
    
    • if critical apply vorporal: insta-kill
    
    
  
  



• Apply Knock Down (if damage > 15): 1d20 + Size (4) + Improved Trip (4) + Str (8)



• 
  
  • Attack of Opportunity: Attack Roll
  
  


• 
  
  • 
    
    • Attack of Opportunity: Damage Roll
    
    
  
  

I would assume anyone could dish out full damage on my character if they just read my card and followed the steps.

This can also be done for defense if needed.

I use a separate scratch note paper.

You could ask the players to write the relevant numbers in a piece of paper before the battle: Which weapon (just a letter or two), which dice, how much to add/subtract, THAC0, whatever the game needs. Same with the relevant often used proficiencies etc, so there's no need to skim a long list to find just the right number. Keep the paper up to date when things change, ask player to add new weapons as they use them etc.

This is scratch paper, so there's no need to be "neat", just cross over old stuff and write new next to it. Therefore this paper is also good for keeping track of current HP and any such rapidly fluctuating resources. Just cross out the old value and write new next to it. There's never supposed to be a lot of current numbers on this paper!

After the session, player might keep the paper for the next session, but it's probably better to just rip it up after the session as a SOP, unless the session needed to end in the middle of a battle or something. This avoids having conflicting numbers on 2 papers at the start of the next session. Ripping it up also avoids the paper accidentally becoming a 2nd character sheet with important information on it.

It shouldn't be long before the players stop needing the paper, but I just think it's nice to have even when you know the rules. It frees my mind for more relevant things than remembering a bunch of numbers, or skimming through long list of proficiencies written with small text. Also, being able to write down loot, names and places etc quick&dirty, then copying it more carefully to character sheet after the session/during my char being out of the action suits me well.

Suggest players use digital character sheets

In addition to Destruktor's suggestions, several members of my group use D&D Beyond to build digital character sheets that automatically do all the calculations for them. That way, they just have 1 number to add to their rolls and all they have to do is know which skill or weapon or spell to look at on their sheet.

They mostly play from their phone or tablet but I think there's a way to print off the character sheet, too.

Different people learn differently

There are other resources available to help them. While Destruktor's answer is the best one, I want to supplement that suggestion with the option of using video instructions from YouTube (Like Handbooker Helper from the Critical Role guys)

There are a lot of basic tutorials out there that may help out, and seeing it done in an instructional video may help people who learn best by watching.

In play, a lot is going on, and it can be confusing for a new player to compartmentalize everything coming at them at once.

What I always say about a gaming system:
"At one point it is very handy to read the rules to play the game correctly and completely."

I wonder what type of people you are playing with because after a year of weekly playing, that would mean 52 game sessions with one or more combats per session with let's assume 4 combat turns. That's about at least 156 times (if they only attack half the turns) they asked what they needed to roll to hit.

So what can you do to help your players?

Ask if they have a copy of PHB. If they don't, they need to download the free PDF rules for DnD Beyond or from WoTC's basic rules.

1. Demand players to explain Combat rules to each-other.

I would demand each player to explain one part of the combat chapter to the rest of the players.

Once you have learned something yourself, by looking into it and figuring it out well enough to explain it to someone else, you will remember a lot better.

If you continue to pamper players, they have no incentive to learn the rules and they will not remember the rules themselves because they haven't made the effort to understand the mechanics.

2. Demand that each player prepares a character sheet

You can check before the session if each character sheet is complete. The most easy way is everybody to make an account on DnD Beyond and so you can see the full sheets and point errors out.

This will also force players to invest time in creating the sheet and reading the rules; or at least the class and features they are interested in.

A lot of the other answers are great but I wanted to add something that I am doing with my new players that hopefully helps them remember why and where these numbers come from.

Explain the "real" world reasons for these numbers

Why do you factor in your Dex modifier when using a finesse weapon?

Why do you add your proficiency to roll?

Explaining where these numbers come from can help them

1. Better understand the system and




2. See the interactions between the character and the math.

Every time one of my players asks what they add to their D20 to hit, if they don't see it already calculated on their character sheet, I encourage them to do the full formula. "Its your Dex modifier plus your proficiency bonus because its a weapon based on how lithe you are and you have experience wielding it."

Make a QuickRules 'Cheat Sheet'

As a new player myself (I'm only level 4 on my first character), I keep thinking what I need is a "game rules" cheat-sheet first not just a specific-numbers cheat-sheet for my character.

I haven't looked for one yet but I'm thinking something like ...

1. 
   

   Skill Check (when asked / attempting to do something in character)

   
   
   
   
   
   • Roll D20 with bonus for that skill (from the character sheet).
   
   
   
   


2. 
   

   First contact at Encounters

   
   
   
   
   
   • DM will ask to roll initiative (roll D20 with initiative bonus from sheet & report to DM when asked).
   
   
   
   


3. Making Attack (Choose which weapon or spell)
   
   
   
   
   • If Weapon
     
     
     
     
     • Declare Target and Weapon
     
     
     • Make Attack Roll (d20 + weapon attack bonus)
     
     
     • If DM says it hit, Make Damage Roll (weapon specifies dice & bonus)
     
     
     
     
   
   
   • If Spell
     
     
     
     
     • Declare Target & spell
     
     
     • Briefly read out spell rules to DM & group
     
     
     • DM makes saving throw
     
     
     • If damage occurs, make damage roll (spell specifies dice & bonus)
     
     
     
     
   
   
   
   

This is probably way too simplistic and even wrong in places, but shows the kind of thing that would help me with what to do when, and tell me when to then go looking for other numbers and which number I am after.

A couple of them have never played before so I understand that they will likely not remember a ton of the rules.

Well that's great because it seems to mean not all your players are new.

I've been playing DnD for maybe 20 years so I've had the opportunity to teach this awesome game to many people.

The best tip I can give you that's always worked is to delegate.

Match your new players with your more experienced ones. If they have a question, they can ask their "mentor" and then only if this person isn't sure of the rule you guys can talk about it. It also opens the possibility of the mentee to ask questions to the mentor while you're doing something else.

Of course, this doesn't exactly fix the fact that your players will know the rules but it will alleviate the burden on your part.

You also need to consider that there are lot of rules in DnD, it can be pretty long before someone is at ease with the game.