Why are Rome's famous concrete structures, such as the Colosseum, still standing after 2,000-plus years, when modern concrete may not...

Let's begin by understanding the meaning of this sentence. We'll use our understanding of pause points and segment the sentence as shown - understanding and assimilating the meaning of each segment bit by bit!

Sentence Structure

• Why are Rome's famous concrete structures,
  • such as the Colosseum,
    • still standing after 2,000-plus years,
      • when modern concrete may not even last for fifty?

• Scientists [?] that
  • the secret to Roman concrete's durability
    • was its unique blend of ingredients,
      • which included volcanic ash and seawater.

Understanding the Meaning

The first sentence sets up an interesting question:

• Why do ancient Roman concrete structures like the Colosseum still stand after more than 2,000 years,
• while modern concrete might only last fifty years?

Now the second sentence gives us the answer. It starts with:

• 'Scientists ______ that the secret...'

This is where we have the blank. Let's look at our choices:

• explain
• having explained
• explaining
• to explain

Now here's what we need to understand. We have:

• Subject: 'Scientists'
• Then we need: a verb to tell us what scientists do
• Then: 'that the secret...' - this tells us what the scientists say

What do we notice about the structure here?

• 'Scientists' is the subject of this sentence
• The sentence needs a complete verb - a main verb that makes the sentence work
• Only 'explain' gives us that complete verb

The other forms - 'having explained,' 'explaining,' and 'to explain' - are all incomplete verb forms. They can't stand alone as the main verb of a sentence.

So we need: explain

Now let's read the complete sentence with this answer:

• 'Scientists explain that the secret to Roman concrete's durability was its unique blend of ingredients, which included volcanic ash and seawater.'

This makes perfect sense - scientists explain (tell us) that the secret was the special mix of ingredients.

GRAMMAR CONCEPT APPLIED

Main Verbs: What Makes a Sentence Complete

Every sentence needs a main verb - a complete verb form (called a finite verb in grammar terms) that can stand on its own. There are also incomplete verb forms (called non-finite verbs) like participles and infinitives that cannot serve as the main verb by themselves.

Complete verb forms (can be main verbs):

• I explain the problem. ✓
• Scientists discover new information. ✓
• The team analyzed the data. ✓

Incomplete verb forms (cannot be main verbs alone):

• Scientists explaining the problem. ✗ (fragment - needs helping verb: "are explaining")
• The team to analyze the data. ✗ (incorrect structure)
• Scientists having discovered the answer. ✗ (fragment - needs main verb)

In our question:

• Subject: Scientists
• Need: a main verb that completes the sentence
• Choice: explain is the only complete verb form that can serve as the main verb
• Result: "Scientists explain that..." = complete sentence ✓