The rules for translating "-ing" words to Spanish are far from straightforward. What is important to remember is the first rule of translation: Translate for meaning rather than word for word. Often, "-ing" words can't be translated directly and require some recasting of the sentence they're in.
With that principle in mind, here are some of the most common way of translating "-ing" words:
As a verb: If an "-ing" word is functioning as a verb, it is probably being used in a progressive tense. Sentences such as "I am studying" and "She was working" are examples of using a progressive tense. In Spanish, progressive tenses are formed in much the same way as in English, by using a form of estar ("to be") followed a gerund (the verb form ending in -ando or -endo). Keep in mind, however, that the progressive tenses are used more in English than in Spanish, so it may be more appropriate to use a simple tense instead. See how the following English sentences can be translated using either progressive or simple tenses:
- He is studying today. Está estudiando hoy. Estudia hoy.
- I wasn't driving a car downtown yesterday. Yo no estaba manejando un carro en el centro ayer. Yo no manejaba un carro en el centro ayer.
- We will be dining in your honor. Estaremos comiendo en su honor. Vamos a comer en su honor. Comeremos en su honor.
- Seeing is believing. Ver es creer.
- Buying on the Internet is a simple process. Es un proceso simple el comprar por Internet.
- Crying doesn't do any good. Llorar no sirve de nada.
- Swimming is the most complete sport. La natación es el deporte mas completo. El nadar es el deporte mas complete.
- The meeting in London ended without an agreement. La reunión acaba sin acuerdos.
- They're fatter because of eating cheap food. Son más gordos por comer alimentos baratos.
- I am thinking about attending a university in the United States. Pienso asistir a una universidad en Estados Unidos.
- I prefer your living here. Prefiero que vivas aquí.
- "Soul" is another way of saying "person" or "living being." "Alma" es otra manera de decir "persona" o "ser viviente".
- I can't hear the snoring man. No puedo oír el hombre que ronca.
- I don't have her forwarding address. No tengo su nueva dirección para reenvío de correo.
- She is a very loving person. Es una persona muy cariñosa.
- There are many people wanting to earn more money. Hay mucha gente que quiera ganar más dinero.
- The princess went away singing because she felt happy.La princesa se fue cantando porque se sentía feliz.
- He studied, thinking about her. Él estudiaba pensando en ella.

