The definition of a substantival phrase is simple: it is a group of words that includes a verb but functions as a noun.
The best way to understand substantival phrases is to see them in action. So let’s look at a passage of Scripture that contains several substantival phrases, Ezekiel 37:1-6. Each substantival phrase is marked in bold.
The hand of the Lord was upon me, and he brought me out in the Spirit of the Lord and set me down in the middle of the valley; it was full of bones. And he led me around among them, and behold, there were very many on the surface of the valley, and behold, they were very dry. And he said to me, ‘Son of man, can these bones live?’ And I answered, ‘O Lord God, you know.’ Then he said to me, ‘Prophesy over these bones, and say to them,O dry bones, hear the word of the Lord. Thus says the Lord God to these bones: Behold, I will cause breath to enter you, and you shall live. And I will lay sinews upon you, and will cause flesh to come upon you, and cover you with skin, and put breath in you, and you shall live, and you shall know that I am the Lord.’
The first five substantival phrases above are are quotations, which are always substantival because they are functioning as the object of the verb of saying that precedes the quotation. The last substantival phrase acts as the object of the verb “shall know” and is connected to that verb with the relative pronoun “that.” We know “that” is introducing a substantival and not a relative phrase because it does not point back to any noun as its antecedent. These are the two main examples of substantival phrases that you will see in Scripture.
On the Phrasing Relationships cheat sheet, there is only one major relationship that a substantival phrase can have with its anchor phrase, marked with the symbol “S.” That relationship is Explanation - Content, and you should label both examples of substantival phrases with this relationship.
Let’s look at both examples in two different passages of Scripture below.
Whenever you have a quotation following a formulaic expression like “David said,” or “Paul said,” they have a Content relationship with their anchor phrase.
As I said above, you can tell that the word “that” isn’t functioning as a relative pronoun because it is not pointing back to a noun. Instead, it is functioning as a marker of content, introducing a substantival phrase. What does the Preacher know? The answer is the direct object of that verb.