The Infinitive

A verb that is not conjugated is, quite simply, infinitive. Infinitive verbs are verbs that either end in -ar, -er, or -ir. There is no use trying to locate an infinitive verb in a sentence that ends in another way. And yes, the only other way this form of the verb will end in a different suffix is when that infinitive verb ends in a pronoun, such as se, le, or lo.

Some examples of verbs, and including a couple with a root change diagram:

detener to stop
volver(o--->ue) to return, to go back
afeitarse to shave(*oneself)

*Notice the reflexive se in the infinitive of the verb above.

These are just one of a myriad ways the infinitive form of the verb exists in the Spanish language. Now, the infinitive is used in a couple of ways. First, it either follows a conjugated verb, or follows a verb phrase requiring an infinitive:

Ella quiere escribir un ensayo breve. She wants to write a brief essay.
Paco y yo tenemos que preparar unas enchiladas. Paco and I have to prepare some enchiladas.

The infinitive also expresses future occurrence using the ir + a + infinitive expression:

Ella va a caminar al mercado. She is going to walk to the market.

Plus, the infinitive has a unique way of showing up on signs, portraying rules or otherwise, when a subject of a sentence is not important to the meaning of it. This is similar to how we use "-ing" endings in the English language. These kinds of signs usually are in all CAPS:

NO FUMAR NO SMOKING
NO ESTACIONAR NO PARKING

The infinitive can also be used to express a noun in its most vague form; when using infinitives like this, you sometimes need to put the singular masculine definite article. In English we tend to omit that. Notice the parentheses around the:

(El) Pasar por acá es prohibido. (The) Passing through here is prohibited.
(El) Hablar en la biblioteca no es común. Speaking in the library isn't common.

You can also use the infinitive with the conjunction a + el (el because the infinitive defaults to the singular masculine) to be the equivalent of the English "upon..." Notice the following:

Al ver el cadaver, la muchacha gritó. Upon seeing the dead body, the girl shouted.