verb + 不了 = impossible action

generally liao is used in phrase, like 了解, means undertand; 了结, means solved; 了得, means awesome. 了: real word, verb: liǎo : understand, see : as in 明了（ming liao)- clearly understand. 了: real word, verb: liǎo : finish : as in 了结(liao jie) - settled/ solved

I think you can consider it as a final particle, to emphasize that something has already happened, you've gotten the result. Compare the following sentences: 他来不了, means "He is not able to come", so the result is clear. 他来了. means "He is coming" or "He has arrived". The result is also clear. 他来不来？Will he come? It's still a question.

no problem! thanks a lot!:)

It's really hard to explain...