Here is everything you need to know about diagramming phrases.
- Prepositional Phrases
It's easy to see that prepositional phrases act as adjectives and adverbs because they begin with a diagonal line coming from the word that they are describing.
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- Gerund Phrases
Gerund phrases always act as nouns. This example is a gerund phrase acting as the subject of a sentence, but the same structure is used for any noun jobs.
To give you a better idea of how they are used, I've also included a diagram of the following sentence:
Running marathons in the summer is hot.

In the above sentence, the word marathons is a direct object.
If you have a predicate noun or a predicate adjective after a gerund, diagram it like this:
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- Participial Phrases
Participial phrases always act as adjectives. They go on a curved, slanted line below the word that they modify.
If the participle has a direct object, diagram it like this:
If the participle has a predicate noun or a predicate adjective, diagram it like this:
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- Infinitive Phrases
Infinitive phrases are versatile! They can act as nouns, adjectives, or adverbs.
This little diagram shows the basic form of an infinitive phrase. Based on how the infinitive is acting (noun, adjective, or adverb), put this little tree in the right space within the sentence diagram.
In this diagram, the infinitive has a subject and a direct object:

If the infinitive has a predicate noun or a predicate adjective, diagram it like this:
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- Appositive Phrases
These act as nouns. The appositive itself goes in parenthesis next to the noun that it is restating, and any modifiers within the phrase should be diagrammed under the noun that they are restating.

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- Verb Phrases
Verb phrases are diagrammed just like other verbs.
