I think it is related to verbs of motion, though I am not an expert on this. Certain verbs with even a small relation to motion need an "a" following the verb. Please consider this response as much a question as it is an attempt to answer the above question.
Here is a list of verbs that need to be followed by "a" at least when an infinitive follows http://www.elearnspanishlanguage.com/grammar/verb/verbswithprep-a.html
It is easier than it seems!
You use "iendo" when the verb (infinitive form) ends in "er" or "ir" and "ando" when the infinitive ends in "ar":
cenar = cenando / saltar = saltando / mirar = mirando
correr = corriendo / meter = metiendo / oler = oliendo
salir = saliendo / partir = partiendo / latir = latiendo
Hope it helps!
The 'ing' ending '-ando' is only used as part of the continuous tense: 'I am walking', 'estoy caminando'. Even though in English you might say, 'you start walking', the second verb 'caminar' must be the infinitive in Spanish. You could also say, 'you start to walk', which is the same as in Spanish.
Khalil, the "a" does not translate to "a" in English; also the lesson is for learning infinitives. I wrote, "You start to walk," and it was accepted. More advanced learners explained that we will be required to include the "a" when translating to español with SOME infinitives, but not all of them, and they provided a study link.
Hello JudeAndre: The following are the imperative forms for empezar: Imperative: tú empieza él/usted empiece nosotros empecemos vosotros empezad ellos/ustedes empiecen. So as "Empieza a caminar" would be imperative for the informal "tu", and the formal Usted imperative would be "Empiece a caminar".