Look at the different ways to express "not enough":
Je n'ai pas assez de lait.I don't have enough milk.
Elle ne court pas assez vite.She doesn't run fast enough.
Il n'est pas assez grand.He's not tall enough.
Not + [adjective / adverb] + enough = Ne ... pas assez + [adjectif / adverbe]
Note that assez is always placed before the adjective / adverb it applies to (unlike in English).
Not enough (of) + [noun] = ne ... pas assez de/d' + [nom]
Note also that de in front of a noun never becomes du, de la, de l' or des. It will however become d' in front of a vowel or mute h.
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