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depuis = can be an adverb meaning "since" or a preposition meaning "since, for.) It is used in expressions such as "I have been in Paris for 3 days = Je suis à Paris depuis trois jours. OR in expressions such as "from Monday to Friday" = depuis lundi jusqu'à vendredi.
Then there is the Present Tense used with "depuis" or "il y a" as in "I have been looking for him for twenth minutes = Je le cherche depuis vingt mnutes OR Il y a vingt minutes que je le cherche.
Then there is the Imperfect with "depuis" or "il y a" or the past tense of what is above. I had been looking for him for twenty minutes = Je le cherchais depuis vingt min utes OR Il y avait vingt minutes que je le cherchais. NOTE: The English can appear to be the present progressive or the past perfect, but it is neither in French!
Other Verbal Expression with duration of time, such as "He works for eight hours = Il travaille pendant huit heures. He worked (for) eight hours = Il a travaillé (pendant) huit heures.
pendant = as an adverb it means "while" in pendant que.....
or it can be a preposition = during
il y a is an impersonal expression that is either singular or plural, meaning "there is" OR "there are."
So, sometimes you can use them interchangeably, sometimes you don't need to state (pendant/pour), it might depend upon what part of speech it is (adverb or preposition) and with expressions having to do with "time" or "duration" see above.