Question 1

Why is it demi-heure without an 'e' at the end of demi, since heure is feminine?

Accepted Answer

This is the exact rule for demi (and nu) when they come before the noun: they are invariable. This means they never change to match gender or number, and they are connected to the noun with a hyphen. If demi came after the noun, it would agree in gender, like in une heure et demie (an hour and a half).

Question 2

If demi is invariable here, why do we use the feminine article une?

Accepted Answer

The article always agrees with the main noun, which is heure (feminine). Even though the adjective demi sits between them and is invariable, the article une is still doing its normal job of matching the feminine noun heure.

Question 3

If this sentence were talking about multiple half-hours, how would we make demi-heure plural?

Accepted Answer

You would only add an 's' to the noun: des demi-heures. Because demi is placed before the noun, it remains completely invariable—it doesn't take an 'e' for feminine, and it doesn't take an 's' for plural either.

Question 4

Why does the sentence use de vibration instead of de la vibration?

Accepted Answer

Here, une demi-heure de acts like an expression of quantity or duration (similar to un verre d'eau or beaucoup de temps). After these expressions, you use de (or d') without the definite article when referring to a general concept or uncountable thing, rather than a specific, previously mentioned vibration.

Une demi-heure de vibration ouvre le chakra.

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