The gerund is normally preceded by the appropriate form of stare/estar. E.g., Sto parlando, Estoy hablando.
There is an important difference, however, between the two languages. In Italian, the gerund always applies to the subject and never to the object. Therefore, a sentence like l'ho visto fumando means that I saw him while I (the subject) was smoking. In Spanish, however, the gerund can apply to the object in certain contexts. The sentence lo vi fumando would normally be interpreted to mean that he (the subject) was smoking.
Created October 2006 by Adam N. Letchford.