"For" and "since" are used to indicate the starting point of an action or a state. We use "for" to express the duration of time, while "since" shows the starting point in time. For example, "I have been studying English for two years" indicates the duration of studying, whereas "I have been studying English since 2018" shows the starting point of the action. Use "for" with a length of time (hours, days, years), and "since" with a specific point in time (Monday, 2010, May).