Difference between 'as long as' and 'so long as'

Question

I am studying English with Understanding and Using English Grammar by Longman.

In an exercise, I have to use given the time words appropriate to the meaning to combine each pair of sentences.

Directions: Combine each pair of sentences. Use the given time words if they are possible and appropriate to the meaning. Cross out inappropriate ones.
In the new sentences, omit unnecessary words, make any necessary changes (paying special attention to verb forms), and punctuate carefully

Below is the question [Q 17 on page 74]. Either a, b, or c needs to join the sentences.

I will not forget Mr. Tanaka. I will live for a long time.

(a) as (b) as long as (c) so long as

The answer is: b. as long as

I will not forget Mr. Tanaka as long as I live.

I don't understand why the 'c. so long as' cannot be the answer, too? I thought the two words have same meaning.

Answer 1

Q: I will not forget Mr. Tanaka. I will live for a long time.

a. as

b. as long as

c. so long as

A: I will not forget Mr. Tanaka as long as I live.

A lot of the difference between as/so long as, depends on the context:

As long as is an adverbial time phrase modifying the clause “I live” = for the length of time that [I live]

As long as I live is an adverbial time phrase modifying the main clause “I will not forget Mr. Tanaka”.

As long as I live = for the length of time that I am alive

Compare:

As long as you hold on to the rope, you are safe. = For the length of time that you hold on to the rope, you will be safe.

So long as is different, in this context, it introduces a conditional adverbial time phrase:

I will not forget Mr. Tanaka so long as I live. = I will not forget Mr. Tanaka, if I am allowed to live / if I am alive

So long as you hold on to the rope, you are safe. = If you hold on to the rope, for that length of time you will be safe.

Answer 2

Table 5 on page 72 of the Longman document clearly states that "as long as" and "so long as" can be used to start adverb clauses with the same meaning- "during all that time, from beginning to end". Both of the examples use the set phrase ""so/as long as I live"".

The Cambridge and Merriam-Webster dictionaries focus mainly on the specific meaning "provided that". The Longman definition is closer to the intended meaning in the OP's sentence, but I think that the dictionaries have correctly identified the more widely used meaning outside of the set phrase.

Looking at the set phrase, this NGram graph shows that "as long as I live" is used about ten times as often as "so long as I live".

One very widely used variant of the set phrase occurs in the wedding vows: most denominations use "as long as we/you both shall live", but the Quaker vow is "so long as we both on earth shall live".

In these two versions of the set phrase, then, it seems that both are grammatically correct, and as long as is the most common.

As long as (but not so long as) is sometimes used outside of the set phrase to specify a time, though it is normal to add the preposition "for", in the same way as you would for any other time specifier. Here is an example:

And then he said she could keep the gun for as long as she liked. - The Migrant Painter of Birds - L Jorge (2011)