Negative conclusion from affirmative premises
For example Open in Syllogism-Finder App:
All squares are rectangles.
All rectangles are tetragons.
Therefore, Some tetragons are not squares.
In principle, no negative conclusions can be drawn from exclusively affirmative (positive) premises. Therefore, a negative conclusion from exclusively affirmative premises is invalid.
When are such conclusions valid
In a certain cases, affirmative statements (especially all-propositions) can be transformed into negative statements (e.g. “all A are B ” is equivalent to “no A is not-B ”). However, this should be done before formulating the inference and not implicitly.