a before the infinitive in the present perfect...???
is a obligatory before the infinitive after the verb in the present perfect? for example....nunca he sabido comer pescado...or... nunca he sabido a comer pescado.
and...when do you use jamas and when do you use nunca....thanks for your attention to these questions....mansoman
jamás and nunca are synonyms, you can replace them and it will make perfect sense.
About the "a".. it is not obligatory.. it depends on the sentence you want to build and the verb on it.
"Nunca he sabido comer pescado" is the correct one (despite it's a weird one :P)
Certain verbs prompt the use of "a" before the infinitive. For example, "He venido a ayudarte." I have come to help you. But it's not, "He querido a ayudarte." It's, "He querido ayudarte." Also, the presence of the present perfect doesn't change anything. Venir + a + infinitive is used in all tenses, as are other verbs that require the "a" in front of a following infinitive. I wouldn't say there is an abundance of verbs that function like this, but there are enough. I would say to google a list. Hope I've helped!
(also, as Denfisksson said, nunca and jamás are synonyms. I've read that jamás is more emphatic, but I am not a native, so I can't say for sure.)
"Venir + a + infinite" is an example of "perífrasis verbales". There are a lot in Spanish. They are two verbs joined with a preposition. At least that is the most usual type. They do not depend on tenses as kraigrd said.