Prefix forms (also called YQTLs or “imperfects”) are distinguished by the fact that they conjugate by adding prefixes, as opposed to affix forms which add suffixes. See the charts in the back of KHW page 432, this video, or this pdf.
Prefix forms are translated into English using the future tense (or sometimes frequentative see Gesenius). However, the prefix form will usually have a vav-conversive attached in which case it is translated into the simple past tense. These forms are often called “preterite”.