WHEN IS THE 'H' SOUND NOT PRONOUNCED IN ENGLISH:

 In general, there is no rule in English to tell you when to pronounce the 'h', but it is rarely silent. So you have to learn the exceptions as they come along. Usually they turn out to borrowings from French, like 'honest', 'honour', 'heir', 'exhaustion', 'Thailand', 'vehicle', 'ghost', 'ghetto' and 'hour'.
The words starting with a silent 'h', if preceded by the indefinate article must have the 'an' version. For example, we'd say 'an honest fellow'. Hope that helps.
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