What is conversion?
Conversion is a type of word formation in which there is a change of a word from one lexical category to another without any morphological form marking the change.
Productivity and creativity
Conversion is an extremely productive way of word formation. The free change from nouns to verbs and vice versa illustrates the creative aspect of the language. (CATCH THE FISH WITH A SPEAR - SPEAR THE FISH)
Constraints on conversion
Conversion is a free process and any lexeme can undergo conversion. But conversion is usually restricted to single morpheme words, and verbs are rarely produced if a word with the intended meaning already exists. There are also constraints on particular subclasses, such as time expressions (holiday, summer etc).
Sources of conversion
Nouns and verbs in English have become identical in form firstly as a result of the loss of Old English verbal endings (CARIAN(v) - CARE(v)/ CARU (n) - CARE (n))
A similar homonymy happened with borrowing French words.
Types of conversion
There are 5 types of conversion: