I see this all the time, and I think I read something about it a some point, but I forget where. Example:

Köyde okul olmadığı için okuyamamış.

Why "okuyamamış"? Why not "okumamış"? What's the difference?

-amamak, -ememek: not be able to, cannot. "-a,-e" the ability, "ma,me" the negativiness suffix, and "mek,mak" are the infinitive suffix.

yapmak: to do
verb stem: yap

yap+a+ma+mak: not to be able to do

Okumak: to study, to read
oku: stem

oku+y+a+ma+mak: not to be able to study (here "y" is put because vowels cannot come one after another)

-abilmek, -ebilmek: to be able to
yap+a+bil+mek: to be able to do
oku+y+a+bil+mek: to be able to study

Meltem's explanation is perfect.
The -e,-a is the 'unabilitative suffix'. Smillarly ve have an 'ability suffix' which is -ebil, -abil.

Some time ago I had written a few exercises on this issue. Of course nobody bothered to try them but it is here on my last messages if you want to have a look:

http://www.turkishclass.com/forumTitle_27_2178_-1

Aaah. I've been doing the negative ability suffixes wrong then. It all makes sense... thanks

And, having read your lesson on this... what I've been doing is confusing the suffix for the ability to not do something with suffix for the inability to do something. Your explanations are very good, erdinç! Thanks!