Since and for is commonly associated to perfect continuous tenses. As explained in the previous chapter of perfect continuous the verbal action has undergone two main functions;
The difference between continuous and perfect continuous
In case of the first sentence the action of jogging is continuous whereas in the second sentence it has been continuous for certain time, and if we use since or for it will be further clear e.g. “They have been jogging for an hour.”
Any action that has been continuous for certain time implies two things; either we have to say for how long it has been continued or when it has been started. It is here that we need since and for.
Point on time means the certain time when verbal action was started. So whenever in perfect continuous tenses we have to talk about the time the verbal action started from we have to use since. There are different types of point of time as follows;
For is used for period of time. It means when we talk about the duration of verbal action, like; two months, four hours, three years, five centuries etc. There are different types of period of time, like; moments, years, months, centuries etc.
Sometimes since and for are used in one sentence where we show both duration and starting time for the verbal action.
Since and for in one sentence
Sometimes one word may stand for both point and period of time. In such cases we have to see what is more likely in accordance with the context.
Position of since and for in a sentence
Position of since and for in a sentence may vary. They can be either at the start or at the end of the sentence, like;