Action in progress:
In Spanish, it is very common to use the simple present tense where the progressive would be used in English. Thus a verb form such as estudio is the equivalent not only of "I study," but also of "I am studying." A conclusion, then, is that the Spanish present progressive is used to emphasize that something is happening now, is in process, or is being repeated. Thus estoy estudiando would mean "I am studying now" or "I am in the process of studying." In some contexts, the use of the progressive might add a nuance of surprise.
Present progressive not used for future meaning:
In English, it is very common to use present progressive forms to discuss future events. Thus "we are eating at 7" is roughly the same as saying "we will eat at 7." In Spanish the progressive would not be used under such circumstances. Say either comemos a las 7 (simple present) or comeremos a las 7 (simple future).
Seldom used with certain verbs:
A number of Spanish verbs typically aren't used in the progressive forms. These include common verbs of motion such as ir (to go), volver (to return) and venir (to come). They also include verbs referring to mental states such as amar (to love) and saber (to know). Thus to ask, "Where are you going?," say "¿Adónde vas?" Another verb normally not used in the progressive is llevar in the sense of "to wear." Thus "he is wearing the blue shirt" would be expressed by "lleva la camisa azul"; the progressive would not be used. Other verbs that are very seldom used in the progressive form are estar, haber (used to form the perfect tenses) and poder (to be able).
One not-always-reliable test you can use to determine whether the progressive is acceptable when translating an English sentence is to add the phrase "in the process of" and see if it sounds awkward. For example, to say someone is "in the process of wearing the shirt" doesn't make much sense, so neither would the progressive in Spanish. But to say someone "is in the process of eating" does make sense, so the progressive in Spanish is possible (although still less likely to be used than in English).
Progressive used to describe prolonged events:
In past and future tenses, the progressive often signifies something that took or will be taking an extraordinarily long time. For example, estuvimos esperando cuatro horas could be used to say "we were waiting for four hours."
Referring to repeated actions:
Sometimes the progressive forms indicate that an action is repeating; this is especially true when used with certain verbs such as ir and venir. Estoy yendo mucho al club. "I'm going to the club often."