Q: I’m confused by the “so … as” and “so … that” constructions in these sentences: “The word is so rare as to be almost obsolete” and “The word is so rare that it is almost obsolete.” Are they both correct? Do they mean the same thing?
A: Your two examples are grammatically correct. The adverb “so,” used to modify an adjective or adverb, can be followed by either “as” or “that.”
These “so … as” and “so … that” constructions can be similar in meaning, though they aren’t identical.
For instance, (1) “Mites are so small as to be invisible” tells us much the same thing as (2) “Mites are so small that they are invisible.” But #2 is the stronger statement, as we’ll explain below.
The difference is clearer when the consequence or result is more stark, as in (3) “The dose was so large as to be fatal” versus (4) “The dose was so large that it was fatal.” Again, #4 is the stronger statement.
Why is this? Because the “so … as” constructions indicate extent or degree, while the “so … that” constructions indicate an actual consequence—in other words, a theoretical versus a real result.
There are grammatical differences as well. The two constructions, “so … as” and “so … that,” require different sentence endings.
In #1 and #3, the preposition “as” is followed by an infinitive phrase (“to be invisible” … “to be fatal”). In #2 and #4, ”that” is followed by a subordinate clause, complete with subject and verb (“they are invisible” … “it was fatal”).
It’s also important to note that in sentences like #1 and #3, “as” is necessary and cannot be omitted. But in #2 and #4, “that” is optional and can be omitted: “The mites are so small they are invisible” … “The dose was so large it was fatal.”
In the #2 and #4 examples, “that” is what the Cambridge Grammar of the English Language would call “a marker of subordination.”
In other words, it marks the subordinate clause that follows. And subordinate clauses don’t always require such a marker.
Both of these “so” formulations are common in negative statements, as in this line from Shakespeare’s The Rape of Lucrece (1594): “No perfection is so absolute, / That some impuritie doth not pollute.”
Sometimes a single sentence combines the two: “It was so hot as to melt concrete, but not so hot that we stayed inside.”
We’ve used adjectival examples here but, as we mentioned above, “so” modifies adverbs as well: “She spoke so loudly as to embarrass us, but not so loudly that the maître d’ asked us to leave.”
from Grammarphobia http://www.grammarphobia.com/blog/2015/10/so-as-so-that.html