=The Tenses=.
The periphrastic or progressive conjugation: "I am buying," "I was selling," "I shall be buying," etc., exists in Spanish with the following differences from English:—
In the present and the past it is used, but only when the action embraces a certain length of time, otherwise the simple form "I buy," "I sold" (imperf. indic.) must be used, as—
Fulano se arrojaba por la ventana (not se estaba arrojando).
The Spanish Academy gives this example as of an action more or less instantaneous: So-and-So was throwing himself out of the window.
The periphrastic form is inadmissible unless one is actually engaged in the action, as:
Hoy como con mi amigo (not "estoy comiendo," because not actually engaged in the action): To-day I am dining with my friend.
In the future this construction is permissible only in such cases as—
Cuando venga mañana, yo estaré escribiendo: To-morrow when he comes, I shall be writing.
The periphrastic form never happens with the verb ir (to go), and seldom with venir (to come).