Grade 3 | English | Preposition (since, for), Preposition and Conjunction, Olympiad, CBSE, ICSE, SOF, ITO
Usages: to denote duration of time, distance or purpose
Time duration: He studied for an hour.
Distance: We walked for a mile to find the shelter.
Purpose: He came here for the drawing competition.
Usages: to denote something happening from the past and still happening. Since is used when we mention the exact point in time when it started.
E.g. They are selling umbrellas since 1980.
Imp concept: For and since are confusing to use in reference with time. Keep in mind "for" is used when you are using a countable unit of time and since is used when you are telling the exact point of time when it started. Try substituting "how many" or "when", if "how many" fits then use for and if "when" seems appropriate use "since".
Incorrect: She has been studying since four hours.
Correct: She has been studying for four hours.