I have had these impulses since I can remember.
1.为何since从句不用since I have been able to remember?
2. 如果从句用since I have been able to remember，和原句相比，二者在句意上有何细微差异？
一、since I can remember 可视为习语，意思是“自我记事以来”。如：
Ever since I can remember, I’ve lusted for a Chevrolet Corvette. 自从我记事儿起，我就一直想要一辆雪佛兰克尔维特。
I have never had this kind of experience since I can remember. 自我记事以来，我从来没有过这样的经历。
I have had such an impression of my hometown since I can remember. 从记事时候起，我对家乡就有这种印象了。
二、remember作为一个“本能”动词，它通常与 can 连用，但一般不与 be able to 连用。如：
As far as I can remember, this is the third time we've met. 我记得这是我们第三次会面了。
They’ve lived here for as long as I can remember.在我记忆中他们一直都住在这儿。
No one got drunk as far as I can remember. 我记得当时没有人喝醉酒。