far, farther/farthest and further/furthest

A         further, furthest

These, like farther/farthest, can be used as adverbs of place/distance:

It isn't safe to go any further/farther in this fog.

But they can also be used in an abstract sense:

Mr A said that these toy pistols should not be on sale.

Mr B went further and said that no toy pistols should be sold.

Mr C went furthest of all and said that no guns of any kind should be sold.

B         far: restrictions on use

far in the comparative and superlative can be used quite freely:

He travelled further than we expected.

far in the positive form is used chiefly in the negative and interrogative:

How far can you see? ~ I can't see far.

In the affirmative a long way is more usual than far, and a long way away is more usual than far away:

They sailed a long way.              He lives a long way away.

But very far away is possible, and so is so/quite/too + far and far + enough:

They walked so far that…                      They walked too far.

We've gone far enough.

far can be used with an abstract meaning;

The new law doesn't go far enough.

You've gone too far! (You've been too insulting/overbearing/insolent etc.)

far, adverb of degree, is used with comparatives or with too/so + positive forms:

She swims far better than I do.             He drinks far too much.