[KCCHAN]

George Orwell's English is excellent, but there is something I cannot understand.

When James Burnham wrote Revolution it seemed probable to many Americans that the Germans would win the European end of the war, and not Russia would dominate the Eurasian land mass, while Japan would remain master of East Asia. This was a miscalculation, but it does not affect the main argument. For Burnham's geographical picture of the new world has turned out to be correct.

In the last sentence of this excerpt from You And The Atomic Bomb, I find the preposition FOR inexplicable;of course we know that it can function as a conjunction, meaning BECAUSE, but how can it be used at the beginning of a sentence?

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Besides, MOST can be a stronger form of VERY, and I wonder whether there is such a sentence:He
is a cleverest person

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Winston Churchill, the former prime minister of Britain, once said...

must the definite article be replaced by an indefinite article?
how about if Churcill is not dead, and is the only former prime minister alive?

THANK YOU