Marx argued that the primary cause of class conflict in capitalist societies is the unequal distribution of wealth, the exploitation of workers by capitalists, and the alienation of workers from their labor.

Engels referred to the unemployed population as the 'reserve army of labor', arguing that they serve as a source of cheap labor for capitalists and help keep wages low.

Marx argued that the ultimate goal of the communist revolution is to establish a classless society, abolish private property, and create a socialist state.

Lenin referred to the Communist Party as the 'vanguard of the proletariat', arguing that it should lead the working class in the struggle for socialism.

Lenin argued that the role of the state in a socialist society is to suppress the bourgeoisie, manage the economy, and provide social services.

Marx referred to the 'dictatorship of the proletariat' as the rule of the working class, arguing that it is necessary to suppress the bourgeoisie and establish socialism.

Engels argued that the state will eventually wither away in a communist society as class distinctions disappear and the need for a state to suppress one class over another diminishes.

Lenin introduced the 'New Economic Policy' in the Soviet Union as a temporary retreat from strict communist economic policies, allowing for some private enterprise and market mechanisms.

Marx argued that class struggle is the driving force of history, as different classes compete for control over the means of production and the distribution of wealth.

Engels referred to the 'materialist conception of history' as the idea that economic factors, such as the means of production and the relations of production, determine social and political development.

Lenin argued that the peasantry, as a class exploited by capitalism, can be a potential ally of the working class in the socialist revolution.

Marx referred to the 'commodity fetishism' as the tendency to treat commodities as having inherent value rather than as social relations, obscuring the labor and exploitation that goes into their production.

Engels referred to the 'dialectic of nature' as the idea that nature is constantly changing and evolving through the interaction of opposing forces and contradictions.

Lenin referred to the 'imperialist stage of capitalism' as the stage of capitalism characterized by the expansion of capitalist powers into new territories, the export of capital, and the division of the world into spheres of influence.

Marx referred to the 'surplus value' as the difference between the value of a commodity and the cost of its production, which is appropriated by the capitalist class as profit.