The present perfect tense denotes present results of past actions.We use" for" when we refer to a period of time.whether you refer to a speaking point of time or no time in the past.
The present perfect tense denotes present results of past actions.We use" for" when we refer to a period of time.whether you refer to a speaking point of time or no time in the past.
eg:-They have investigated (a case) for a period of time of 15 days.