Understanding of the sentence structure

Question

It is unbelievable that such conditions could have existed so short a time ago

Could anyone help me understand the structure of this sentence in particular the highlighted area?

Answer 1

It is unbelievable
   that such conditions could have existed
           so short a time ago.

What is probably giving you trouble is:

so short a time ago

and you would probably not have any trouble if the sentence said so long ago.

ago = "in the past"

so long ago = "so far back in the past"

We can say so long ago (adverb long exists) but we cannot say so short ago (adverb short being non-existent), so we must inject "a time" into that phrase:

so short {a time} ago

such a short time back in the past, i.e. "so recently".

Answer 2

Consider the following sentence -

It's impossible that we can make it within an hour.

Notice that in the sentence above within an hour is a temporal adjunct. It's realized by a Preposition Phrase (PP), headed by a preposition - within. The head preposition takes a complement in the form of a Noun Phrase (NP) - an hour - that sits after the head preposition.

Inside a PP structure a head preposition can either take a complement or they don't. When prepositions take complements, we call such preposition Transitive Preposition. Similarly, when a preposition takes no complement, we call that preposition Intransitive Preposition.

Complements can be in various form. It can be a NP, an Adverb Phrase (AdvP), an Adjective Phrase (AdjP), another PP or even a clause.

until recently [Preposition - until Complement - recently, an AdvP

within an hour [Preposition - within Complement - an hour, a NP]

Note that not all prepositions can license every form of complement. That means, some preposition can license AdvP, but some don't. Some licenses a clause, others don't.

Normally the complement comes after the head preposition. But there are some very limited number of prepositions that allows its complement to come before it, and not after it. One example of such preposition is - ago.

In your example sentence -

It is unbelievable that such conditions could have existed so short a time ago.

The part - so short a time ago - is a Temporal Adjunct, that is realised by a PP. The preposition - ago - takes a complement in the form of a NP - so short a time. But notice that the preposition - ago - allows its complement to come before it. Take the complement after the preposition, and it would be ungrammatical.

Now the complement of the preposition is a NP - so short a time. Notice the structure of the NP. Inside the NP structure the head noun is - time, and the AdjP that modifies the head noun is - so short. The determinitive that occurs inside the NP is - a. There are some AdjP that comes before the determinitives. So the NP structure is correct - so short a time.