The letter "A" before a verb

Question

When is the letter "A" used in front of a verb

Accepted Answer

Hola Peter 
There is a large amount of verbs in Spanish that require "a" + infinitive", but unfortunately there is no specific rule to know which verbs follow this, it is random. There are lots of verbs using a specific preposition, the same way there are these verbs in English, for example: to consist of..., to talk about..., to get used to... etc; these are prepositional verbs. 
We do have a list with some common prepositional verbs (different prepositions, not only "a"), have a look here. However we have plans to publish a list with verbs following "a + infinitive", for example: ir a + infinitive, ayudar a + infinitive, comenzar a + infinitive, etc.. It will be published soon.
Gracias y saludos
Inma

Answer

For advanced students, there is an interesting and useful book published by Espasa Calpe entitled "Diccionario de uso de las preposiciones españolas" by Emile Slager - which gives prepositions used after various verbs (and nouns and adjectives): it is quite a comprehensive listing. Often the correct choice depends on context, and the text actually gives examples of sentences using the cited prepositions, with specific references as to who actually wrote them, and when.