Last year it was raining one day, therefore I took my umbrella to work. The rain got heavy and thereby delaying my JR train by 30 minutes. It was very irritating so thereafter I started taking my car to work.
These expressions can be confusing and are often a source of mistakes but here are some examples to explain how to tell the difference between them.
Therefore means = because of/ as a result.
It was raining therefore I took my umbrella→ Because of the rain I took my umbrella.
The train was canceled therefore I had to walk → the train was canceled, because of that I had to walk
Thereby means = by doing that.
The man stood in front of the door thereby blocking the entrance → The man stood in front of the door and by doing that he blocked the entrance.
I turned on the air conditioner thereby making the room cooler → I turned on the air conditioner and by doing that I made the room cooler.
Thereafter means = after that , following, then.
I studied at a university and thereafter I worked as a businessman → I studied at a university and then I worked as a businessman.
I got a good discount on pizza at dominos so thereafter I started buying from them → I got a good discount on pizza at dominos so after that I started buying from them.
While confusing at first using these expressions can give variety to your English!!!