These two expressions look very similar, so it is only natural that they are easily confused. Perhaps the easiest way is to first look at the basic meanings.

"As far as" means "to that limit of distance, and no further". For example if the road we are travelling on is blocked, I might say to you:

This is as far as we can go.

If we are standing on the top of Mount Fuji, I can say:

There is a carpet of cloud as far as the eye can see.

"As long as" means "to the limit of that time, and no longer". Here are some example sentences:

I will love you (for) as long as I live.
He tried to put off the meeting (for) as long as possible.

With this meaning of "as long as", we can use "for", as shown by the brackets in these two sentences.

These meanings are not that difficult. The difficulty arises in expressions such as those given as the title of this Column. In the next Column, I will show how:

"as far as" is related to the concept of "limit" (限界)
"as long as" is related to the concept of "if" (条件)

As I said in the previous Column, "as far as" is related to "limit"; "as long as" is related to "if". I will give some sentences to show this, and also give Japanese translations in order to make the point clear:

As far as I know, he is Chinese.
(私の知っている限りでは、彼は中国人です)
As far as I am concerned, you are wrong.
(私に言わせてもらえば、きみは間違っている)
She lives in Osaka, as far as I am aware.
(私の知っている限りでですが、彼女は大阪に住んでいます)
Her name is "Mary", as far as I can remember.
(私が覚えている限り、彼女の名前は"Mary"です)

"As long as" has a rather emphatic or strong conditional meaning, very similar to "provided".

As long as [Provided] you finish your homework, you can watch television.
(宿題が終わっているのだったら、テレビを観ていいよ）
We can go by taxi, as long as [provided] you have enough money.
(十分な金さえあれば、タクシーで行けます）
I will phone you, as long as I can remember the number.
(電話番号を覚えていれば、電話します）

As an interesting extra point, notice that "as long as" can be ambiguous, as in:

I will love you as long as you love me.

This can have a conditional meaning (あなたが私を愛してくれるのなら、私はあなたを愛します). It can also mean "to the limit of time" (あなたが私を愛しているうちは、私はあなたを愛します) [see previous column]. If we want to make the second meaning clear, we have the choice of saying " ... for as long as you love me".

Conditional uses are a bit difficult for learners of English. It is certainly difficult to keep the "balance" between the clauses:

If it rains, I will take an umbrella.
(雨が降れば、傘を持っていくつもりだ) [降るかどうかは微妙]
If it rained, I would take an umbrella.
(雨が降れば、傘を持っていくのだが)[降りそうもない]
If it had rained, I would have taken an umbrella.
(雨が降ったら、傘を持っていったのだが)[降らなかった]

These three patterns are the basic three patterns of conditional sentences.

Then, as is well known, there are special uses with "were". In this Column, and the next one, I will discuss "were" vs. "was".

Should we put "was" or "were" in the following blank space:

A: They say that there was an earthquake yesterday.
B: Well, if that (　　) so, I didn't feel it.

The answer is "was". But what about this sentence:

If I (　　) a bird, I would fly to you.

The answer, as everyone knows, is "were", although "was" is also correct in informal English. The earthquake situation is clearly about something that happened (or did not happen) in the past. The bird situation is not related to time in any way. It is a general statement.

Some people (including those who write various kinds of exams) think that "if" is a kind of switch, which automatically switches on "were". This is not the case. I will continue this point in my next Column.

The following sentence, which I found in a book, is about surveying the length of a meridian in France in 1797:

If one side of one of these triangles were known, the lengths of all the others could be calculated.
(これらの三角形の一辺がわかれば、他の二辺の長さが算出されえた)

The story is about what people did and what they wanted to know more than two hundred years ago. It should be "was known".

But let us imagine that you and I are at this very moment sitting down, trying to work out a geometry problem. In such a case, I can certainly say to you:

If one side of one of these triangles were known, the lengths of all the others could be calculated.
(これらの三角形の一辺がわかれば、他の二辺の長さが算出されうるのだが)

The "time" here is "non-time", or "imaginary time", or "wishing time". It is the same "time" as the "time" in, "If I were a bird, I would fly to you".

But what happened, and what people wanted to happen, in France in 1797 is past time.

Here is another example, which I found in very well-known British magazine, and have slightly rewritten to make it clearer:

Leigh Hunt wrote as if he were walking with the reader, arm in arm.
(レイ・ハントはあたかも読者と腕を組んで歩いているように執筆した)

This is about how Hunt wrote more than 150 years ago. It should be "was walking".

Since subjunctive uses have been dying out in English for a very long time, it is only natural that there is a degree of confusion about them. Also, "were" tends to sound elegant or educated, therefore writers are tempted into using it even when "was" is the correct form.