In Continental Portuguese (which appears to apply in Brazil as well) the 'a' before the verb implies the continuous tense. Examples herewith. Eu estou a comer = I am eating. Eles foram a estudar = They were studying. But I am a complete beginner so perhaps you should treat my comments with extreme caution Andy. Regards - Mark Lahure
Have I missed something? An old Living Language course book asserted "certain verbs are followed by a preposition before an infinitive. Some require a; others take de, em, or por. There is no rule for determining which preposition follows any verb." AFAIK, the a is there because 'começar', along with 'aprender', 'ajudar', 'aconselhar' and 'continuar' are some common verbs requiring an a before an infinitive. I am surprised if Andy_G's answer was rejected, as I would have thought it just as valid as the one Lahure supported ( Example Ela pode tentar caminhar → 'She can try walking' or 'She can try to walk')
I think that the use of estar + a + infinitivo (EuPt) and estar + gerúndio (BrPt) is different from Duo's sentence. They are both ways of translating the English present continuous tense.
ex: Ele mantém o otimismo, mas sabe que está correndo contra o tempo. = Ele mantém o otimismo, mas sabe que está a correr contra o tempo. (He maintains optimism but knows that time is running short.)
Ele começou a correr. = He started to run. // He started running. (Very slight difference of meaning, but nothing to do with the present continuous tense)