A Guide To Parts Of Speech


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An infinitive consists of the root of a verb plus the word "to" (called

an "infinitive particle").   Beginners confuse the infinitive particle

"to" (which should always precede an infinitive) with the

preposition "to." The bad news is that they are not related, so

writers simply need to make a mental note that there's another

word out there spelled t-o, and it means something else.  We

manage this with plenty other words:  fly; nail; lead; and so on.

As with other verbals, infinitives are like verbs in many ways, but

they are not verbs as parts of speech.  The first, and most important way, in which they are different from verbs

is that they are not affected by tense.  As the name implies, they exist outside of time and duration altogether, in

an eternal "now"--infinitely, as it were.  The following familiar quote is from William Shakespeare's Hamlet (Act

III, Scene i).  All of its infinitive phrases have been emboldened, while all of the infinitive verbals have

additionally been underlined:

"To be, or not to be: that is the question:

Whether 'tis nobler in the mind to suffer

The slings and arrows of outrageous fortune,

Or to take arms against a sea of troubles,

And by opposing [to] end them? To die: to sleep;

No more; and by a sleep to say we end

The heart-ache and the thousand natural shocks

That flesh is heir to, 'tis a consummation

Devoutly to be wish'd. To die, to sleep;

To sleep: perchance to dream.

Shakespeare's tragic hero is here caught in a moment of existential crisis over fate and eternity.  Here, the

infinite that the character ponders is aptly shown through Shakespeare's use of the infinitive verbal form: taking

Hamlet and the audience "outside of time" for just a while.  This is exactly the effect that the infinitive verbal form

creates.  In this way, it very closely resembles the gerund in that it's a way of being, rather than an action.

However, infinitives do not imply an activity in progress the way that gerunds do.  (And, this is logical because

infinitives do not draw from any progressive verb tense the way gerunds do.)  In fact, if we extend the photo

"snapshot" analogy to gerunds and infinitives, a gerund freezes the action in time whereas an infinitive shows a

still-life; a  gerund captures an activity in progress while an infinitive abstracts from it a concept.  A gerund

captures a behavior while an infinitive conveys a principle.  No wonder Shakespeare paired infinitive verbs

with one of the most philosophically important soliloquies in his collected dramatic works!

Infinitives can assume the role of three different parts of speech:  nouns, adjectives, and adverbs. Additionally,

infinitives can take an object or a complement if they are not derived from intransitive verbs.


Infinitive phrases are not difficult to diagram, but they can be a little tricky because you have to identify first

which part of speech they're playing.  Also, because they can take predicates and complements, they have to

be set up like clauses without subjects, and too many horizontal lines can make for what seems to be a lot of

clutter in a diagram.

Don't worry. With a little bit of practice and diligence, you'll get the hang of them before long.  Following are

examples and discussions of three different kinds of infinitive phrases:

Noun Infinitive Phrases

The way to determine if an infinitive is behaving as a noun is to ask the basic questions "What? or  "Who?"

because, if a noun is a person, place, thing, or concept, then an infinitive phrase behaving as a noun can also

be any one of these:

To excel as a writer requires patience and practice.

What requires patience and practice?  To excel requires these.  "To excel as a writer" answers the question

"What?" and is therefore a noun infinitive phrase.  Furthermore, "to excel" is derived from an intransitive verb,

so, in this case, it doesn't take a predicate object.  However, it still is modified in the way that intransitive verbs

are, so "as a writer" is an adverbial phrase answering the question, "How to excel?"  This makes "as a writer"

bound to the infinitive "to excel" as a modifying phrase, and all of it comes together to creates one single

infinitive phrase:  "To excel as a writer..."

Because an infinitive noun phrase comes with an entourage of modifiers and sometimes predicate objects and

complements, it can take up so much real estate on a horizontal line that the "raised platform" method is

needed (the same one used for gerund phrases).  Think of it as a car mechanic lifting up a car so that it can be

worked on from underneath.

Adjective Infinitive Phrases

The way to identify if an infinitive is behaving as an adjective is to determine whether it answers the adjective

questions "Which? or  "What's it like?" because, if an adjective modifies a noun, then an infinitive phrase

behaving as an adjective must also modify one or more of these in a sentence:

The urge to produce offspring in a safe environment leads many species to migrate large distances.

There are two infinitives in the sentence above.  Only one of them is an adjective. First, the verb in the main

clause, "leads," is causative:  one action causes another by way of an indirect object.  If "many species" is the

indirect object, then the infinitive phrase "to migrate large distances" is the direct object.  By nature, all objects

are nouns or words acting like nouns, so, in this case, "to migrate" is a noun infinitive.  The remaining infinitive,

"to produce offspring in a safe environment" answers the question "Which urge?" The answer is "The urge to

reproduce" so this infinitive and its predicate object are all part of a single adjective infinitive phrase.

Because an infinitive adjective phrase doesn't need to go on a horizontal line, it doesn't use the "raised

platform" method that infinitive noun phrases use.  Instead, they are placed under the nouns they modify,

connected by diagonal lines the way other modifiers are.  Your first instinct will be to say that "many species" is

the direct object and "to migrate large distances" answers the question "How does it lead them."  And you'd be

right.  The causative verbs can be thought of in this way as well, but, again, it's merely convention that they

must be rendered with indirect objects and prepositional phrases as direct objects.  Another way to think of the

infinitive phrase "to migrate large distances" is that it answers the question, "What are many species lead to

do?"  Phrased this way, we expect the answer to be a noun, and that's what we get:

Adverb Infinitive Phrases

Adverb infinitives are the most difficult to classify, even though they're not difficult to recognize.  This is because

the common adverb has, not one, but three modifying functions.  It can modify verbs, as in "walks further"; it

can modify adjectives, as in "takes a slightly inconvenient route"; and it can modify other adverbs, as in "walks

even further."  The way to identify if an infinitive is behaving as an adverb is to determine if, in the sentence, it

answers the adverb questions "How?", "How much?" or  "When?"


Emergency Room physicians struggle to prevent their own depression and burnout.

The infinitive phrase, "to prevent their own depression and burnout" answers the question "Struggle how?"

"Struggle" is an intransitive verb, so a word modifying a verb is always going to be an adverb.  The infinitive "to

prevent" is derived, not from an intransitive verb, but a transitive one, so it takes a predicate object, "depression

and burnout."   The entire infinitive phrase, then, modifies the verb "struggle" and is therefore adverbial.


Forced to make difficult life-and-death decisions, an E.R. physician is more prone to depression.

The infinitive phrase, "to make difficult life-and-death decisions" answers the question "Forced how?" "Forced"

is a past participle modifying "E.R. physician."  A past participle is a kind of adjective, so a word modifying an

adjective is always going to be an adverb.  The infinitive "to make" clearly modifies an adjective and is

therefore an adverbial infinitive phrase.

For a change, the rules for diagramming an adverbial infinitive modifying an adjective are simple and intuitive.

No, really.  Just extend a zig-zag line from the diagonal line containing the adjective, and "hang" the infinitive

phrase from it as you would a bird-feeder.  Then, sit back and enjoy whatever wisdom the birds start bringing.


Just as an adverb modifying an adverb is far less common, an adverbial infinitive modifying an adverb is less

common, but it happens enough to warrant mention.  In fact, the sentence I just wrote contains an adverbial

infinitive phrase modifying an adverb:  "it happens enough to warrant mention."  The word "enough" is an

adverb modifying "happens," and "to warrant mention" modifies the degree of  "enough."  Any word modifying

an adverb must, itself, be an adverb, so an infinitive phrase modifying one must be an adverbial infinitive.

Here's another example:

Many progressive social programs aren't funded sufficiently to make any real difference.

As can be seen from the example, the infinitive phrase "to make any real difference" follows on the heels of the

adverb "sufficiently."   It answers the question "How sufficiently?" which is typical of most adverbs modifying

other adverbs:  they describe degree or intensity.  Words like "more," "less," "enough," "adequately" and, yes,

"sufficiently" are the most commonly occurring of these.

The rules for diagramming an adverbial infinitive phrase that modifies an adverb are virtually no different from

those governing an adverbial infinitive modifying an adjective.