A few questions regarding economics

I think you're equating "value" with "price", these are not the same thing. (Exchange value is also another thing). "Value" refers to the amount of socially necessary labour time needed to reproduce a commodity, what that level of SNLT is, is regulated by market exchange.Food is a commodity if it is produced with a view for sale on the market. If I grow food in my garden without the intent on selling, it is not a commodity.How much you need to differentiate between different types of commodity would depend on what you were trying to do. I suppose.You might like to watch these videos:https://kapitalism101.wordpress.com/

Yes, in some of his earlier writings on economics (his A Contribution to the Critique of Political Economy that came out in 1859, for instance), Marx did not make a distinction between "exchange value" and "value" but in the various editions of Capital (from 1867 on) he did. As YMS has just pointed out, "exchange value" is the expression of "value" in exchange, i.e is the two are not the same. As we said in this Education Document, dating from 1973:

So, value is a social relationship (between commodity producers as producers producing something for sale) while "exchange value" is a relationship between things.When money has evolved "exchange value" is the same as "price". So, if you don't distinguish between "value" and "exchange value", you end up with only use value (utility) and price, like modern academic economics which preaches that you don't need the concept of "value".It also leads to misunderstanding the Marxian Labour Theory of Value as a a Labour Theory of Exchange Value (or Price) as it was for Adam Smith and David Ricardo.  Which leads to all sorts of complications and contradictions.As a matter of fact, Marx did not think that under capitalism, where there is not just production for exchange (the market) but there is also production for profit (surplus value), commodities were priced at their (exchange) value. Due to the tendency for the rate of profit to be the same in all fields of capitalist investment, commodities were priced at what he called their "price of production", which was cost price + average rate of profit. Only accidentally would this be the same as the amount of socially necessary labour needed to produce them from start to finish.  But, as items of wealth produced for sale, they were still expressions of the underlying social relationship of "value".

I am linking two seminal passages from Karl below. I think the general idea is that prices do vary according to supply and demand, but as our marketers are always telling us they are always gravitating towards an ‘equilibrium’. But how and why by what force? The answer using the labour theory of value is not only simple but it is also an answer. When demand exceeds supply you can sell it at above its labour time value and make a bigger than average profit. And when supply exceeds demand you have sell it at below its labour time value and make a below the average rate of profit. It is simpler to see directly and first in simple commodity production as Rubin laid out somewhere but I will use my own example. If I am a artisan in a village making new fangled and popular  blue suede shoes I can sell the product of my 20 hours of labour for what most people have to work 40 hours for [making their own commodities and selling it] The goose egg market in the same village for example might be over supplied and the goose herders have to work 60 hours. Some goose herders change profession and turn their hand to blue suede shoe making driving down the remuneration for blue suede shoe making; and increasing the remuneration for goose egg production for the ones that stick with it. Until a price equilibrium is reached until what can be got for 40 hours worth of blue suede shoes is the same as what can be got for 40 hours worth of goose eggs. The same thing works in capitalism except all capitalist insist on getting on getting 50 hours worth of stuff for 40 hours of paid labour or whatever.  And as far as workers are concerned selling their own only commodity, labour power, lorry drivers become computer programmers etc.   Value, Price and ProfitVI. Value and Labour  So far the market price of a commodity coincides with its value. On the other hand, the oscillations of market prices, rising now over, sinking now under the value or natural price, depend upon the fluctuations of supply and demand. The deviations of market prices from values are continual, but as Adam Smith says: “The natural price is the central price to which the prices of commodities are continually gravitating. Different accidents may sometimes keep them suspended a good deal above it, and sometimes force them down even somewhat below it. But whatever may be the obstacles which hinder them from settling in this center of repose and continuance, they are constantly tending towards it.” I cannot now sift this matter. It suffices to say the if supply and demand equilibrate each other, the market prices of commodities will correspond with their natural prices, that is to say with their values, as determined by the respective quantities of labour required for their production. But supply and demand must constantly tend to equilibrate each other, although they do so only by compensating one fluctuation by another, a rise by a fall, and vice versa. https://www.marxists.org/archive/marx/works/1865/value-price-profit/ch02.htm    Capital Vol. III Part IIConversion of Profit into Average ProfitChapter 10. Equalisation of the General Rate of Profit Through Competition.Market-Prices and Market-Values. Surplus-Profit  https://www.marxists.org/archive/marx/works/1894-c3/ch10.htm  Actually in capitalism it can get even more complicated as what happens when a new fangled labour saving method of making something springs into existence. If 5% of the stuff is being made by the new method and 95% by the old what is the real socially necessary labour time to make something in the inevitably slow transition period from one general method to the other? He dealt with that elsewhere in Volume III as the market [labour time] value I think. 

Value is the form of wealth under capitalism, or exchange economies based around socially necessary labour time.

Not bad. This would be a bit better:"Human labour, applied to materials provided by nature, is the only thing that can create wealth (in capitalism this takes the form of "value"). For example, a leather shoe can only exist because someone spent labour time getting all the materials, sewing the shoe so that all the parts stay together."And just to complicate things, not all labour in capitalism produces "value". Some sectors just redistribute value that has already been created in other sectors of the economy. See the thing and productive and non-productive labour.

The best way to learn 'Marxian economic is by reading the proper sources and bourgoise economists are not able to explain how the capitalist economic operates.During the crisis of the 2008, many academic economists ( econometrists ) had to go to the libraries or the bookstores to get Marx's Capital because using their own academic knowledge they  were not able to explain or understand what was happening around the world.The Socialist Party of Great Britain have written many articles that explain and answer all the questions and issues that have indicated in this thread, and also the Socialist Party of Canada have a pamphlet that explains in details all this questions.  PD More tha 75% of the work that we are doing at the present time is unproductive