El gerundio y el infinitivo

A. THE GERUNDIO is used for actions in progress (present participle in English):

A1. with the appropriate form of estar (or hay) to form the progressive tenses:

Lisa está estudiando.
Estamos aprendiendo.
No hay nadie hablando.Lisa is studying.
We are learning.
There’s no one talking.

  Note that in Spanish this construction cannot be used to express the future, as it frequently is in English (I am leaving next week). The simple present can be used in this sense: “Salgo la próxima semana”.

A2. with the verbs seguir and continuar to mean "go on doing something":

Continuó diciendo.
Siguió cantando.He went on saying.
She continued to sing.

   Note that, unlike English, Spanish does not offer the option of using an infinitive after seguir or continuar:
He went on to say → Siguió diciendo.

A3. to express "by doing" or "while doing" something (no preposition in Spanish):

Vas a mejorar estudiando mucho.
Me duermo leyendo esto.You are going to improve by studying a lot.
I fall asleep (while) reading this.

 Spanish does not use the gerundio as an adjective.Clauses or different expressions have to be used:

the answering machine
a never-ending process
passengers carrying a big case...
an intriguing beginning
a disappointing endingla máquina contestadora (el contestador)
un proceso que nunca termina (interminable)
los pasajeros que llevan una maleta grande...
un comienzo intrigante
un final decepcionante

 B. THE INFINITIVE is the verb form that Spanish uses as a noun:

B1. as the subject of a sentence or the object of a verb*:

Ver es creer.
No me gusta estudiar.
Decidir casi siempre es difícil.Seeing is believing.
I don't like studying.
Deciding is difficult most of the time.

B2. immediately after any preposition:

Voy a salir después de comer.
Es su manera de hablar.
Habla sin usar las manos.I am going to leave after eating.
It's his/her way of speaking.
He speaks without using his hands.

 Note that, unlike English, Spanish does not use thegerundio after prepositions or as the subject of a sentence. The infinitive must be used in such cases:
the problem of leaving → el problema de salirliving is good → vivir es bueno

*  Many verbs require no preposition before the infinitive: Quiero dormir.
Some, however, require different prepositions: Sueño con viajar. Ayudan a mejorar. Tratamos de entender. (See Appendix D).

Más ejercicios aquí.