text: (1)I have been a secretary for five years. \n\n'am' is present tense and 'for five years' is a past tense duration\n\n(2) I saw him yesterday.\n\n'have seen' is used for a present statements about the past as in 'I have seen a grown man cry' whereas 'saw' directly relates to the past\n\n(3) She's liked him for six months.\n\n'since' refers to a starting point in the past whereas 'for' in this case refers to the duration 'six months'