Do I use the ability mod or skill mod when rolling an ability check?

Question

When rolling for an ability check for say stealth, do I use the DEX modifier(say +3) or the stealth modifier(say +7) or both?

Answer 1

(All references from D&D Player’s Basic Rules v0.2)

In fifth edition you don't have a stealth modifier per se.

When you make an ability check, you add your ability modifier (Dex) but if you are proficient in the relevant skill (Stealth) then you also add your proficiency bonus.

Sources:

Page 4

Roll the die and add a modifier. Roll a d20 and add the relevant modifier. This is typically the modifier derived from one of the six ability scores, and it sometimes includes a proficiency bonus to reflect a character’s particular skill.

and

Page 7

Your proficiency bonus applies to many of the numbers you’ll be recording on your character sheet:

• Ability checks using skills you’re proficient in

(This is being nitpicky about language, but allows us to use the same terms as the rulebook uses).

The sum of your ability modifier and proficiency bonus should be what is written next to the skill. This is likely the +7 you refer to in the question, which is effectively your skill bonus, but that isn't a term used by the rules.

Answer 2

All skill checks are Ability checks in 5th edition, however, most character sheets include a section for skills and saving throws. These fields are just compilations of the bonuses that are included for ease of reference.

Your stealth skill, for example, is a Dexterity check so you use your Dexterity modifier when you roll a check (+3 in your case). If you are proficient in this skill, you gain your proficiency modifier to those checks (+2 until you reach level 5).

From the looks of the numbers you gave, your character is probably a rogue. They have a special ability that allows them to double a proficiency modifier for chosen skills. This means instead of the +2 proficiency modifier, you have a +4 modifier for checks with that skill.

So your attribute modifier (+3) and your expertise/proficiency modifier (+4) gives a total bonus of +7 to stealth checks. The character sheet has this already calculated for convenience, so that is what you see next to the skill.

Dex Mod  +  Prof Mod (with expertise)  =   Total bonus
  +3                +4                 =       +7

Thus, you only receive the +7 modifier to those checks, but you can calculate them quickly even if they aren't written in (and will need to recalculate them when you raise your dexterity or reach level 5).