Meaning of "不了" in "忘不了/喝不了/..."
了 here is read liǎo which means:


  to finish / to achieve / variant of 瞭｜了 / to understand clearly   -CC-CEDICT


thus 

不了


  unable to / without end -CC-CEDICT


忘不了 = can't forget / unforgettable

喝不了 = can't drink / undrinkable