Affix forms (often called QTL forms or “perfects”) are distinguished by the fact that they conjugate by adding suffixes, as opposed to prefix forms which add prefixes. See the charts in the back of KHW page 432, this video, or this pdf.
Affix forms should usually be translated into a simple past tense verb or as an English perfect. Sometimes, these verbs will have a vav-reversive and should be translated as futures.
See the syntax of the perfect form explained in this video or in this document.