Infinitive

To sneeze, to smash, to cry, to shriek, to jump, to dunk, to read, to eat, to slurp—all of these are infinitives. An infinitive will almost always begin with to followed by the simple form of the verb. An infinitive will almost always begin with "to." Exceptions do occur, however. An infinitive will lose its "to" when it follows certain verbs. These verbs are feel, hear, help, let, make, see, and watch.

Basic Background information on the term

In the English language, the infinitive is basically just the simple root form of a verb, prefaced by the word "to." So, the infinitive form of the verb "write" would be "to write." 

In many non-English languages, including the Romance languages derived from Latin, the infinitive tends to be a single word. For example, the Spanish "escribir" would be the equivalent of the English phrase "to write". 

Understanding infinitive's proper usage and rules

Here is an example of the infinitive being used correctly within the context of an actual sentence. 

"According to one book that many people have found inspiring, the purpose of human life is to eat, to pray, and to love." 

In this sentence, the last three verb phrases are in fact examples of the infinitive. 

Now, here is an example of the incorrect use of the infinitive. 

"In the morning, the family will to go camping, as has been their Labor Day weekend tradition for many years now."  

In this sentence, the verb "go" should not be in the infinitive but rather in a conjugated form.

In case you are still a little confused about the infinitive, here are a couple rules you can follow in order to make sure you are using it in a correct way. 

1. The infinitive is the original, non-conjugated form of a given verb. It can show up in this form within the context of certain kinds of sentences. What is far more common, though, is to see some conjugated form of the verb, and not the infinitive itself. 
2. Again, in English, the infinitive form simply consists of the root form of the verb with the word "to" before it. So, when you see this formulation, you are most likely looking at an infinitive. 

Infinitives founded in the romance languages

The infinitive form is somewhat more clearly seen in the Romance languages than in English. In Spanish, for example, almost all verbs, in their unconjugated form, consist of a single word ending in -ar, -er, or -ir. This would be the infinitive form of each of the verbs, and it is generally where the student of Spanish begins when trying to learn the language.

In English, on the other hand, the word "to" must be added before the verb itself in order to form the infinitive. A distinction is thus sometimes made by linguists between the bare infinitive which consists of only the verb itself, and the full infinitive which includes the word "to" as well.   

The most salient feature of the infinitive is probably just that it can be conjugated in any number of ways. Although the infinitive itself can serve in important function within sentences, its real value comes from this grammatical resilience: beginning from the infinitive, the language user can proceed to conjugate the verb into any of the tenses available in his language, include all past, present, future, progressive, and perfect tenses. The infinitive is fundamentally defined by the fact that it is the only form in which the verb is completely unconjugated: it is the verb itself, independently, in its root form.