I would like to know the difference between 'buy and purchase'. IS there any rule to use these words?
As far as I know purchase is used for non-living things only where buy can be used with living thing e.g persons.
Make correction if I am wrong.
Their meanings are exactly the same and neither is restricted in terms of what is bought (past tense of buy).
Purchase is a slightly more formal verb, but can also be used as a noun:
a purchase = something that is bought
The noun purchase has another meaning: for example, rubber soles give better purchase on slippery surfaces - purchase here means grip.
Hope this helps.