What role does 事 play in 事不師古？
事不師古而能長久者﹒非所聞也


事 : working, you may shortcut it to governing in this context

不 : not

師 : learn, or imitate

古 : past

而 : and

能 : can be

長久 : long time

非: not

所聞: heard, maybe recorded

so, the verse is roughly:

[if the] governing (事) is not (不) imitate (師) to the past (古), and (而)[such governing] can (能) [last] long time (長久), it's unheard (非所聞).

that 淳于越 suggested that nations not using feudalism, would not last long.

some underlying asumptions:

in ancient china, both 商 dynasty (bc 1571 - 1046)  and  周 dynasty (bc 1046 - 770) used feudalism. compare to spring and autumn period (春秋), and warring states (戰國) period, these two dynasties lasted 400-500 years, which was a long time.

the related text in 秦始皇本紀, would started from "始皇置酒咸陽宮"


  仆射周青臣進頌曰：「他時秦地不過千里，賴陛下神靈明聖，平定海內，放逐蠻夷，日月所照，莫不賓服。以諸侯為郡縣，人人自安樂，無戰爭之患，傳之萬世。自上古不及陛下威德。」


then one of the master 淳于越 refuted:


  博士齊人淳于越進曰：「臣聞殷周之王千餘歲，封子弟功臣，自為枝輔。今陛下有海內，而子弟為匹夫，卒有田常、六卿之臣，無輔拂，何以相救哉？事不師古而能長久者，非所聞也。今青臣又面諛以重陛下之過，非忠臣。」


last, the prime minister 李斯 suggested:

丞相李斯曰：「五帝不相復，三代不相襲，各以治，非其相反，時變異也。今陛下創大業，建萬世之功，固非愚儒所知。且越言乃三代之事，何足法也？異時諸侯并爭，厚招游學。今天下已定，法令出一，百姓當家則力農工，士則學習法令辟禁。今諸生不師今而學古，以非當世，惑亂黔首。丞相臣斯昧死言：古者天下散亂，莫之能一，是以諸侯并作，語皆道古以害今，飾虛言以亂實，人善其所私學，以非上之所建立。今皇帝并有天下，別黑白而定一尊。私學而相與非法教，人聞令下，則各以其學議之，入則心非，出則巷議，夸主以為名，異取以為高，率群下以造謗。如此弗禁，則主勢降乎上，黨與成乎下。禁之便。臣請史官非秦記皆燒之。非博士官所職，天下敢有藏詩、書、百家語者，悉詣守、尉雜燒之。有敢偶語詩書者棄市。以古非今者族。吏見知不舉者與同罪。令下三十日不燒，黥為城旦。所不去者，醫藥卜筮種樹之書。若欲有學法令，以吏為師。」

you may treat the text as argument of "feudalism" vs "totalitarianism"

the text maybe further elaborated, if you need :)

have fun :)