The foot of "but soft!" is iambic.
A trochaic foot consists of a stressed syllable followed by an unstressed syllable. An iambic foot consists of an unstressed syllable followed by a stressed syllable. The word 'but' is unstressed, and the word 'soft' is stressed. The exclamation mark at the end suggests that the last syllable is stressed. Other examples of iambic words are 'unite' and 'provide.'
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