Why can't "I have a new hat." be chapeau neuf or nouveau chapeau?

Hi Christopher, 

Having checked, both 'nouveau' and 'neuf' are accepted so as Maarten suggests it depends on what emphasis you choose to highlight.

Bonne Continuation !

Christopher, 

some things are best explained as being ‘right’ because they are what would be said by a native speaker. Maintaining grammatically correct language use is not the only standard applied.

Just as it is usually unnecessary and therefore uncommon in English to say ‘ I bought a brand new shirt/dress/shoes/hat ‘ instead of just saying ‘ I bought a new shirt etc ’,  a native French speaker will usually and naturally say ‘ un nouveau chapeau ‘.

The ‘norm’ ( and hence implied message) is that some things, such as a hat, will be ‘ brand new ‘, although not necessarily your first ever purchase of such item. Additional information can and usually would be given if it is not ‘new new’ !

The adjective ‘ neuf ‘ is used to emphasise ‘brand new’ but does not need or have to be used when the assumption is that something is unlikely to be 2nd hand.