We use since and for to refer to time. They are used to refer to an event that began in the past and continues to the present.
In English, we use since to refer to a point of time. Since can refer to a point after a specific time or event in the past. Since refers to a particular point of time beginning sometime in the past and continuing until the present time.
Point of time can be:
Name of the day like Sunday, Monday
It can be particular year like 2021
It can be exact time like 5 O’colck
Just remember that since is used to refer to a point of time.
As since refers to a point of time, a specific time in the past which constitutes to the present, we cannot use it in future tense.
Here are two examples in the present perfect continuous tense:
She has been teaching since 8 a.m.
I have been watching television since 10 p.m.
For is used with a period of time that is indefinite period of time.
I had lived here for four years.
He has been sleeping for two hours.
Just like since, for is used to refer to an action that began in the past and continues to the present but for can be used with other verb tenses also, including the future.
He will be out of the city for two months.
She is joining us for one week.
For vs. Since
Remember, for is used with a period of time that is indefinite period of time.
Since is used to refer to a specific point in time that is definite point of time.
You can use for and since with similar verb tenses, if you wish. Here are two examples:
She has been working for two hours.
She has been working since 9 a.m.
They have been living here for 10 years.
They have been living here since 2001.
In the example sentences, both for and since show an event that began in the past and continues to the present.