Correct usage of Prepositions: of, about, for, with, by.
Of expresses the relationship of a part of something to its whole. It is the most used preposition in English.
- He is a boy of 15.
- Some parts of his body were injured.
- Most of the guests are gone.
- The plays of Shakespeare will always be classics.
About means ‘on the subject of’ something or ‘concerning of’ something/someone. It also means ‘approximately.'
- I was about to leave.
- He cares about his classes.
- I have about 30 candies in my bag.
- This article is about computers.
For means ‘with the object or purpose of’. It indicates suiting of purpose for something/someone.
- I will always be here for you.
- I have done it for my benefit.
- Cucumber is a good vegetable for making a salad.
- Take my gratitude for your kind hospitality.
With means ‘accompanied by’ something or someone.
- I will always be with you.
- I have brought some candies with me.
- He is playing with his sisters.
- It is free with this product.
By means ‘as a means of’ something or someone. This preposition often comes in a passive sentence.
- Do you travel by bus/train/plane/bike?
- He was undone by me.
- The letter was written by him.
Source: Learn Grammar