Worth a read: the word 'worth'

Recently [22 March 2015], a columnist in the weekly English column of a local Hong Kong newspaper discussed a common error involving the word ‘worth’.  The writer noted that many Hong Kong English users consider ‘worth’ to be a verb, as in the following sentence:

[1] *This painting worths a lot of money. [INCORRECT]

The correct way of expressing this sentence in English is:

[2] This painting is worth a lot of money. 

Though the writer was quite right to call this an error, his explanation of why was less convincing. 

The problem, he claimed, was that ‘you cannot add “-s", “-ed", or “-ing" after the word “worth"’, which to him suggested that 'worth' here in sentence [2] is actually an 'adjective'.

It is true that 'worth' is not a verb, but it is not an adjective either. Mostly in English it functions as a preposition (like through, under, over etc). 

Worth as a preposition

How do we know 'worth' is a preposition? Well, in English, the rule is that prepositions cannot be followed by a 'that' clause (like "he claims that he has a degree') or an infinitive clause (like 'he claims to have a degree'). The word 'worth' follows exactly this rule, showing that it behaves just like other prepositions. We CANNOT say:

[3] *The effort of cleaning the elderly centre was worth that we spent three hours getting our hands wet. [INCORRECT]
[4] *The stunning night view of Hong Kong is worth to visit.  [INCORRECT]

On the other hand, prepositions CAN be followed by a noun phrase (I hope you make it through the day), by an '-ing' clause (I will call you after sending this email), and by a relative clause beginning with 'what' (Let's make some notes about what you did today). The word 'worth' behaves in just the same ways:

[5] The smiles we get from the patients are worth a thousand sleepless nights. [followed by a noun phrase]
[6] The spectacular Hong Kong light show alone is worth watching. [followed by an '-ing' clause]
[7] Many say that the third runway is not worth what the taxpayers will have to pay for it. [followed by a relative clause starting with 'what']

Coming back to our original example, it matches the pattern of sentence [5]. In fact you will most commonly see 'worth' used directly after the verbs 'is' or 'are':

[8] My watch is worth $25,000.
[9] These magazines are worth your attention.