Page 35 - Practical English Usage 3ed - Michael Swan, Oxford

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I can see a ship. (More natural than I am able to see a ship.)
Be able is used in cases (e.g. future, present perfect) where can/could is not grammatically possible because it has no infinitive or participles (see 121.1d).
One day scientists wiU be able to find a cure for cancer.
(NOT ••• wi"ea"Jind...)
What have you been able to find out? (NOT Ylhat Mfle}'6lt e6ltitl ...?) I might be able to help you. (NOT I might Cd" ..•)
Able is not often followed by passive infinitives.
He can't be understood. (More natural than He's not able to be understood.)
For differences between could and was able, see 122.5, 123.2.
about and on
Compare:
- a book for children about Africa and its peoples a textbook on African history
- a conversation about money a lecture on economics
We use about to talk about ordinary, more general kinds of communication. On suggests that a book, talk etc is more serious, suitable for specialists.
about to
About + jnfinitive (with to) means 'going to very soon'; 'just going to'. Don't go out now - we're about to have lunch.
I was about to go to bed when the telephone rang.
Not about to can mean 'unwilling to'.
I'm not about to pay 100 dollars for that dress.
above and over 'higher than': above or over
Above and over can both mean 'higher than'. Above is more common with this meaning.
The water came up aboVe/over our knees.
Can you see the helicopter above/over the palace?
'not directly over': above
We use above when one thing is not directly over another.
We've got a little house above the lake. (NOT ••• 6~ the ltJlee.) 'covering': over
We prefer over when one thing covers and/or touches another. There is cloud over the South o f England.
He put on a coat over his pyjamas.
We use ouer or across (see 9) when one thing crosses another. The plane was flying over/across Denmark.
Electricity cables stretch ouer/across the .fields.
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