1. During what period was the working day increased, and why?
From the mid-14th century to the 1830s; because capital was developing and 'breaking in' the working class.
2. During what period was the working day reduced, and why?
From the 1830s onwards. 'The limiting of factory labour was dictated by the same necessity which spread guano [fertiliser] over the English fields', but more importantly by 'the working class movement that daily grew more threatening'. Despite orthodox academic claims that Marx predicted ever-increasing physical misery for the working class, Marx is here describing the working class progressively improving its lot even within capitalism - though later chapters will show what ways the capitalists find to circumvent and nullify those working-class gains.
3. Marx's analysis depends on making a sharp distinction between human labour and the labour of other animals. If a farmer can produce more by e.g. using horses, Marx regards that as an increase in the productivity of the farmer's labour, rather than a surplus gained by the farmer exploiting the horses' labour. Why?
Human labour is a deliberate social process, not just an instinctive activity. 'At the end of every labour-process, we get a result that already existed in the imagination of the labourer at its commencement'. Marx's labour theory of value is a theory of how value relates to and is based in (a particular form of) human society. Not nature but society makes labour-time appear as value.
4. What are constant and variable capital? And why does Marx call them that?
Constant capital is capital embodied in means of production and raw materials. Variable capital is capital embodied in labour-power. When the capitalist uses constant capital, it only transmits the given value to the products, bit by bit. When the capitalist uses variable capital, he or she sets in motion living labour, and thus adds greater value to the products. The value laid out on variable capital is the 'variable' component of capital.
5. Why can't machines and raw materials add more than their own value to the product?
Because value is a social, not a natural, property. The value of a commodity is a mechanism whereby a social relation is established between the labour employed on producing it and other labour. Labour cannot count for more in this mechanism simply by virtue of being added at an earlier stage of the total production process (i.e. in producing some intermediate product) rather than at a later.