You are asking for the negative form (as opposed to the opposite of it):
“He certainly hasn’t done it.”
…………………………………………
Why can we not say, “I am working here since 2000.”
‘am working’ is the Present Progressive tense. Think about the time sequence implied in this conversation:
SHE: Hurry up. I need to use the bathroom.
HE: I won’t be long. I am/I’m just brushing my teeth.
Can you see that the ACTION referred to – brushing – had already begun; was continuing as HE spoke; and ended after HE spoke.
This can be depicted as:
………|S>……….am…|NOW|>….brushing…..>E|…………
………|S>……….am…|NOW|>….working…..>E|…………
where S = start of the action
NOW = the moment of speaking the words
E = end of the action
‘since’ refers to a PERIOD OF TIME, starting in the past, and up to NOW, the moment of speaking.
………|Since>……>|NOW|….….…………
Can you see that the tense of the verb clashes with the period of time covered by the conjunction ‘since’: the verb form takes us BEYOND NOW, whilst ‘since’ only covers UP TO NOW.