A YQTL verb (also called "prefix forms" or “imperfects”) conjugate by adding prefixes (cf. QTL). See this video or here.
YQTL forms are translated into English using the future tense. However, it will usually have a vav-conversive attached in which case it is translated into the simple past tense. These forms are often called “preterite”. see here.
The syntax of the prefix form is explained in this video.