how do Marx and Engels understand the motor of change in human history? How do they view the role of class?

they thought that history's struggles were always between the "haves" and the "havenots" that would rise up in jealousy or an attempt to gain equal conditions. they also believed that eventually, the "proletariats"/havenots would eventually rise up and overthrow the haves completely and institute a global age of happy communism

relies upon on who you're asking! in case you're asking a functionalist, then little or no, in case you're asking from a marxist attitude then its the completed foundation of human historic past and historic materialism. In Feudal circumstances the land vendors controlled the serfs and used their labour as quickly as you get into the commercial age then classification conflict is the completed foundation of the communist manifesto. The labour of the proletariate is used and exploited via the capitalist vendors of the flexibility of production. in accordance to Marxists you could not have human historic past without sophistication conflict