Progressive Tenses

The progressive tense indicates an action that is ongoing.  It is formed by using estar (in any tense) with a present participle.  Present participles are formed by dropping the ending of the verb, and adding the following endings to the stem:

Present Participles

-ar	-ando
-er	-iendo
-ir	-iendo

Juan está hablando.  Josh is talking.
Estaban cantando.  They were singing.
Estuve escribiendo una carta.  I was writing a letter.

A few irregular present participles: poder - pudiendo, dormir - durmiendo, sentir - sintiendo, venir - viniendo, pedir - pidiendo, leer - leyendo, ir - yendo, reír - riendo.