The progressive forms in English are those made by following a conjugation of the verb "to be" with a present participle, that is, the "-ing" form of the verb. Thus the following sentences (with the progressive form in boldface) demonstrate use of the progressive form: I am drinking the milk. We will be leaving at 8. He was thinking about her.
The progressive form, sometimes known as the continuous form, in Spanish is very similar in terms of structure. It is made by following a form of the verb estar with the present participle, that is, the form of the verb that typically ends in -ando or -iendo. Here are some examples of its use: Está nevando. Pedro está comiendo el taco. Rosita estará leyendo el libro. (It is snowing. Pedro is eating the taco. Rosita will be reading the book.)
Although the Spanish progressive looks like the English progressive and is structured much the same way, there are some key differences. Following are explanations of the Spanish progressive, including the ways in which it differs from English:
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 corollary, 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. (But see the paragraph below about repeated actions.) 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).