[susan53]

Sorry - didn't see the first part of the question before.

For has the same meaning as because, yes. But it is different syntactically. Because is a subordinating conjunction, and the subordinate clause which it introduces must always be attached to a main clause, whatever order they are in. So I can write both:
He was promoted because he was so good at his job.
and
Because he was so good at his job, he was promoted.
but I can't write : *He was promoted. Because he was so good at his job.

Notice I said write - in spoken English the main clause often gets "detached" from the subordinate clause. As in :
A : Why was he promoted?
B: Because he was so good at his job.

For, however, is not a subordinating conjunction but a co-ordinating conjunction - like and, but and or. And these are often used to start sentences. Look at what I've written here (before and after this point) and you'll find two examples of but starting a sentence, two of and and one of or.


And if you don't trust me, here's Barack Obama in the Yes we can speech :

There will be setbacks and false starts. There are many who won't agree with every decision or policy I make as president, and we know that government can't solve every problem. But I will always be honest with you about the challenges we face. I will listen to you, especially when we disagree. And above all, I will ask you join in the work of remaking this nation the only way it's been done in America for two-hundred and twenty-one years--block by block, brick by brick, calloused hand by calloused hand.

Later in the speech you get :

To those who seek peace and security - we support you. And to all those who have wondered if America's beacon still burns as bright--tonight we proved once more that the true strength of our nation comes not from our the might of our arms or the scale of our wealth, but from the enduring power of our ideals: democracy, liberty, opportunity, and unyielding hope.

For that is the true genius of America--that America can change. Our union can be perfected. And what we have already achieved gives us hope for what we can and must achieve tomorrow.

As for your question about "cleverest" - I'm not sure what you're asking, but if it's about the article : "the" is always used with the superlative, so "*a cleverest" is not possible.

Or were you asking about the form of the superlative? Two syllable adjectives (egnarrow, modern, clever) often have two ways of forming the comparative and superlative - with er/est or more/most. So you could use both, eg :
He's the cleverest person I know.
or
It was the most clever thing I've ever heard.