So the verb has a negative declension, or is it just some kind of conjunction that negates the verb?

You repeat the verb but in the positive or negative form if that makes sense?

Like “Do you understand?” is “I understand”.

I’m teaching my youngest lad Irish and I’ll regularly ask him “An dtuigeann tú” (on diggin too) and he’ll respond with “tuigim” (“tigim” - “I understand” (which is strictly a contraction)).