Strange "A" Appearing Before Subject Pronouns
Why does a random "a" appear at the beginning of some sentences? It isn't the personal a, as far as I can tell, because it doesn't come before a direct object.
Ex: A él le gusta este concierto.
2 Answers
Just for clarification, the 'a' is not appearing before a subject pronoun.
Let's look at your sentence........
A (preposition) él (indirect object - him) le (indirect object pronoun him/her/it) gusta (conjugated verb present tense - to be pleasing) este concierto (subject - this concert).
The 'a él' is not needed, it is added for emphasis. Say for example there are two people ( a boy and a girl) at the concert and you wanted to point out that he is enjoying it.
A literal translation in English would be .... this concert is pleasing to him. But of course, we would simply say......He likes this concert.
This is neither strange nor random. In your example it emphasizes that he likes this concert.
For emphasis you can always add the "a + pronoun" in a sentence. See bosquederoble's clarification in the comments.
a mí (note written accent) = me or I
a ti, a usted = you
a él, ella = he / she or him / her
Look up pronouns that come after a preposition for further examples.