If you are completing Form 1040A then pension distributions are entered on line 12 or line 16 if you are completing Form 1040. A 1099-R is issued for a distribution from a pension, profit sharing, 401(k), annuity, or IRA. Gross distribution is the amount in box 1 of Form 1099-R but is not necessarily the taxable amount. The taxable amount may be indicated in box 2a. If the taxable amount is less than the gross amount, then the gross amount is listed on line 12(a) or 16(a) and the taxable amounts are listed on lines 12(b) or 16(b). If the entire gross amount is taxable then that amount can be entered on line 12(b) or line 16(b).
If the distribution is from an IRA then lines 11 or 15 are used.