“Spun” and “span” are both past tense forms of “spin”, but “span” is both archaic and singular, and so would be an odd choice in any current writing, and a disagreement in number where, as here, it is used with a plural subject.
What do you mean singular? I span, you span, he/she/it span, we span, you span, they span. These all sound grammatical to me. I can’t think of many English verbs that inflect differently for the same person but different number. In fact off the top of my head the only one I can think of is “to be” (I am, we were)
There's nothing about it being singular in the dictionary. And I've seen both used in modern writing. Perhaps a US vs UK vs rest of the english speaking world thing?
