* Present perfect is used to show an action that happened in past at an unparticular time.
Example: she has worked here.
* It is used with SINCE and FOR. for the time specification.
Since: is used for the exact period of time i.e exact day, exact date, exact time, exact year etc (since sunday, since july, since 2010, since 10:00 AM)
* For: is used for the duration of time.
example: for ten years, for two hours, for two days, for ten months etc.
* Also Present Perfect is used to repeated actions.
Example: I have eaten at ABC restaurant twice/thrice/many times/acouple of times.
Structure: sub+Has/Have+3rd form of the verb+object
* Simple past tense is used to show an action that happened in the past, finished in the past, and the time is clear.
Example: I went to school yesterday.
Structure: Subject+2nd form of the verb+Object
P.S: The main recognition of these two sentences is according to their structure, in PRESENT PERFECT we use 3rd form of the verb (gone, eaten, slept, written) etc and in SIMPLE PAST we use 2nd form of the verb (Went, ate, slept, wrote) etc.