- I think she's sad.
- I think she's unhappy.
- I think she's not happy.
- I think she isn't happy.
- I do not think she's happy.
- I don't think she's happy.
(...) each version strikes us as a bit weaker or milder in e-negativity or negative force, or in the degree to which the speaker is committed to the embedded subject's unhappiness, than the one immediately above it.
(Horn, A Natural History of Negation, p. 316)
Such paradigms reflect the interaction of two general principles: (a) negative force weakens with the distance of the negative element from the constituent with which it is logically associated, and (b) negative force strengthens with the degree of incorporation (or morphological absorption) of the negative element.
ひとつめの原則 (a) は，否定辞繰り上げに関わり，ふたつめの原則 (b) は接辞否定・否定辞編入に関わります．たとえば：
a. I think I didn't see anyone in the room.
b. I don't think I saw anyone in the room.
c. I think I saw no one in the room.
いちばん上の例文 (a) は中立で，(b) は否定が弱まり，(c) では否定が強まるそうです．