Am I correct in thinking one uses demi when talking about something with an indefinite article and moite with a definite article?

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Accepted Answer

Hi Graham,
As Chris has already pointed out you will find examples of demi and moitié used with both definite and indefinite articles. I can see where the confusion occurs as in English you only have one word - half.
Demi will usually be used with other words except when you ask for a beer in a bar : Je voudrais un demi s'il vous plaît.
But normally it will be used before the noun it refers to to mean half of it, une demi-heure, une demi-bouteille, une demi-journée, une demi-portion, un demi-verre ... e.g. J'ai fait un demi-marathon, l'hotel est à formule demi-pension.
Moitié is a noun and here are some examples:
J'ai fini la moitié de mon travail , j'ai lu la moitié de mon livre, nous avons mangé la moitié du gateau ( you cannot use demi in that instance - moitié meaning half of that whole).
There are some interesting expressions using demi and moitié , like à demi /à moitié   which mean the same thing, half done.
e.g. une bouteille à demi pleine, un pot à demi plein , un mur à moitié peint, un travail à moitié fini
'Ma moitié' which means my better half, my soul mate, my spouse.
Hope this helps but not easy !

Answer

Hi Graham, I don't think you can slice it that way. In my understanding it is perfectly OK to say, "Donnez-moi cette demi-baguette."
The distinction is with respect to using it as an adjective (demi/e) or a noun (la moitié).
Une demi-page est rouge. -- A half-page is red.Une moitié de la page est rouge. -- One half of a page is red.
There are foten used colloquialisms and appreviated forms evolving around demi.
Il est six heures et demie. -- It is half past six.This is an abbreviation of "...six heures ete une demi-heure".
On se renconte à la demie. -- We are meeting at half-past. (This assumes an agreement about which hour one is talking).
Je voudrais un demi. -- I would like a half-pint of beer. (This works only colloquially  in a bar when ordering beer.)
-- Chris (not a native speaker).

Answer

Darn, again the typos....:
foten --> often.appreviated --> abbreviatedete --> et
-- Chris.