‘Since’ and ‘for’ are used in the Present Perfect tense, where we mention the period of an action that started in past and is still continuing at present. Let’s understand the difference between these words with the help of an example.

Start of rain(5 Pm) 7pm(Now) Still Raining

Let’s assume that it started raining at 5pm and now it is 7pm and it is still raining. In other words it has rained for two hours and we have to indicate the period of this process. For this we have two options.

For – When we have to mention the period of the action. In this example it is two hours. Other examples are three days,five months,six years etc.
Since- This used when we have to the time when the action started. So it is meant for the point of time. In this example it is 5Pm. Other examples are 1st October ,Yesterday,December,Childhood etc.

There are two ways to describe the above mentioned process.
It has been raining for two hours.
It has been raining since 5Pm.