https://www.englishclub.com/grammar/..._for-since.htm
http://www.grammar.cl/Notes/For_Since.htm

In short,

For is used to describe the duration of time e.g for two days ,for two hours ,etc.

Since is used to describe the distance between two points of time from past e.g since 9.0 clock,since 1981.since yesterday etc

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The pessimist complains about the wind,the optimist expects it to change,the realist adjusts the sails.