"They have been unable to determine who kidnapped him"= "They are unable to determine who kidnapped him" are same sentences? please explain
In the first, their inability to determine who kidnapped him began in the past. In the second, we know only that they are unable to determine this now.
In practical terms, there is not a great deal of difference between the situations spoken of in these two sentences, though only the first can suggest that they have tried for some time.