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:
 *This painting worths a lot of money. [INCORRECT]
The correct way of expressing this sentence in English is:
 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  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:
 *The effort of cleaning the elderly centre was worth that we spent three hours getting our hands wet. [INCORRECT]
 *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:
 The smiles we get from the patients are worth a thousand sleepless nights. [followed by a noun phrase]
 The spectacular Hong Kong light show alone is worth watching. [followed by an '-ing' clause]
 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 . In fact you will most commonly see 'worth' used directly after the verbs 'is' or 'are':
 My watch is worth $25,000.
 These magazines are worth your attention.
About this blog
This blog arises from keeping an eye on English in Hong Kong. I often use signs, notices and advertisements that I see as starting points to write about English issues that commonly challenge Hong Kong writers.