Infinitive

English

The infinitive is composed of two words: to + the dictionary form of the verb.
For example: to speak, to dance.
Although the infinitive is the most basic form of the verb, it cannot be used in a sentence without another verb which is conjugated.
For example: To learn is exciting.   In this sentence to learn is the infinitive and is is the conjugated verb. 
The dictionary form of the verb, rather than the infinitive, is used after such verbs as let, must, should and can.
For example: Mr. Smith let his daughter drive his new car.  Note you do not use the word to before drive.

Spanish

The infinitive is only composed of one word, the verb. 
For example: hablar (to speak), comer (to eat), vivir (to live)
The infinitive form is always used for a verb that follows any verb other than ser (to be), estar (to be) or haber (to have).
John and Mary want to dance together.
Juan y María quieren bailar juntos.
I can leave tomorrow.
Puedo salir mañana.
You should study more.
Usted debe estudiar más.