About and On
Both about and on can mean ‘regarding’. There is a slight difference of meaning.
- We had a discussion about money.
- He gave a lecture on finance.
About used in the first sentence suggests that the discussion was ordinary. On used in the second sentence suggests that the lecture was serious or academic, suitable for specialists.
Above and over
Above and over can both mean ‘higher than’.
- The water came up above / over our waist.
Above is preferred when one thing is not directly over another.
- There is a temple above the lake. (The temple is not directly over the lake.)
Over is preferred when one thing covers or touches another.
- He put on a sweater over his shirt. (NOT He put on a sweater above his shirt.)
In measurements of temperature and height we use above. In measurements of ages and speeds we use over.
- The temperature never rose above 5 degrees Celsius.
- You have to be over 18 to see that film.
Across and through
The difference between across and through is similar to the difference meaningon and in. Through is used for movement in a three dimensional space, with things on all sides. Across cannot be used with that meaning.
- We went through the wood. (We were in the wood.)
- The road goes through the forest.
- We walked across the desert. (We were on the desert.)