How we cite our quotes: (Chapter.Section.Paragraph) or (Chapter.Paragraph)
Quote #1
Labour, then, as the creator of use-values, as useful labour, is a condition of human existence which is independent of all forms of society; it is an eternal natural necessity which mediates the metabolism between man and nature, and therefore human life itself. (1.2.6)
Marx doesn't often say that something is a universal condition always present in all times and places. Usually, he goes on about how capitalism or feudalism or other modes of production—and the ideas that go along with them—are just temporary phases in history. But here he says the need to work is something common to all societies, whether they're communist or capitalist or something else.
Quote #2
Use-values like coats, linen, etc., in short, the physical bodies of commodities, are combinations of two elements, the material provided by nature, and labour. If we subtract the total amount of useful labour of different kinds which is contained in the coat, the linen, etc., a material substratum is always left. This substratum is furnished by nature without human intervention. When man engages in production, he can only proceed as nature does herself, i.e. he can only change the form of the materials. Furthermore, even in this work of modification he is constantly helped by natural forces. Labour is therefore not the only source of material wealth, i.e. of the use-values it produces. As William Petty says, labour is the father of material wealth, the earth is its mother. (1.2.7)
Labor needs raw material to work with. That's why natural resources, such as oil or coltan (necessary for the increasing quantity of cell phones), play such a crucial role in world events, including war. A society can't create wealth unless it has raw material to work with. And if a society can't create wealth, people aren't occupied with work, and, jobless and despairing, they might cause civil unrest or revolt (source).
Quote #3
The value-form of the product of labour is the most abstract, but also the most universal form of the bourgeois mode of production; by that fact it stamps the bourgeois mode of production as a particular kind of social production of a historical and transitory character. If then we make the mistake of treating it as the eternal natural form of social production, we necessarily overlook the specificity of the value-form, and consequently of the commodity-form together with its further developments, the money form, the capital form, etc. (1.4.17)
Marx is saying there's nothing natural about capitalism or any other mode of production. Societies can change economic systems. If we mistakenly think capitalism is eternal and natural, as other economists of Marx's day did, we start making errors in how we analyze it.