The difference is very simple indeed. We use 'for' followed by a period of time (how long?) and we use 'since' followed by a point in time (from when?) . Here are some examples, using the present perfect tense. I've lived here for 10 years. ('10 years' is a period of time) I've lived here since 2010. ('2010' is a point in time) He's been on holiday for three weeks. He's been on holiday since January 12th. Clear? Here's a little exercise for you. Decide whether the words after the gap describe a period or time (like 10 years) or a point in time (like 2010) and put in the either the word 'for' or 'since': We've been married ........... three years. I've worked here ............ two months. I've been ill .............. last Wednesday. She's been waiting .............. five minutes. They have been going out together ........... December. I've played the piano ............... I was a small child.

An adverbial phrase which begins with "for" means that something continues during a certain period of time. ／On the other hand, an adverbial phrase which begins with "since" means that something has continued from a specific point in time. ／e.g. It is 2020 this year. I have lived in the United States since 2016. I have lived here for 4 years. ／since 2016 = starting from 2016, for 4 years = from 2016 to 2020

For • period/amount of time Since • a specific point in time I have been thinking about your question for 2 hours. I have been thinking about your question since I woke up.