During late summer evenings, we would sit on the pouch, swing and rock very slow.

Explanation

This is the error. The adverb "slowly" should be used instead of "very slow." Adverbs typically end in -ly, and in this context, "very slowly" is the correct form. Adverbs modify verbs, and in this case, "slowly" is the correct form to modify/describe the verbs/activity "swing" and "rock."So, the corrected sentence is: "During late summer evenings, we would sit on the pouch, swing, and rock very slowly."