Some verbs in Portuguese have to be followed by a preposition just like some verbs in English have to take a preposition in order to make sense. It is not just ‘a’ but also ‘de’, ‘com’, ‘em’ etc …
If you have a reasonably good bilingual dictionary, it will give examples of how to use the word. For instance, in my dictionary when you look up aprender, it gives the example ‘aprender a ler’ which tells me the normal construction is ‘aprender a + verb’. However the construction for learning a subject would be ‘aprender + noun’.
So for example, ‘Vou aprender Português’ but ‘vou aprender a tocar piano’.
The trick is to learn verbs with any preposition that usually follows it e.g. começar a; pensar em etc
This becomes even more important when the same verb changes meaning slightly or even a lot sometimes depending on the preposition that follows it
e.g. ‘sonhar com + noun’ (to dream about something) and ‘sonhar de + verb’ (to dream of doing something).
You will see also that ‘dream’ is an example of how English uses prepositions with some verbs.
Hope you didn’t mind me jumping in and you found this helpful.