Use of A to start a sentence

In lesson 9.8 there is a sentence:  “A Roberta piace l'America.”
Why does it start with “A”?  Why doesn’t it say. “Roberta piace l’America”?

Hi cedch, 

Thanks for your question!

The verb "piacere" always needs the preposition "a", as in "A Roberta piace l'America", or "A me piace sciare".  Alternatively, you can find the pronouns "mi" (to me), "ti" (to you), "gli" (to him), "le" (to her), "ci" (to us), "vi" (to you), "loro" (to them).

It's important to note that the structure of this verb doesn't literally translate from the English verb "to like".

Perhaps it's best thought of as “to please, to be pleasing to” rather than ‘to like’; in fact, in Italian it’s the object or activity that ‘pleases’ you.

For instance: 
"A Roberta piace sciare" (Roberta likes skiing) could be thought as "Skiing pleases Roberta"
or
"Le piace sciare" (She likes skiing) 

Again:
"A Marco piace la torta" (Marco likes the pie) could be thought as "The pie pleases Marco"
or
"Gli piace la torta" (He likes the pie)

Hope this helps!