Compra is the third person singular of the present tense of comprar.
So it means:
Conjugation of comprar in the present:
Because comprar is the infinitive, meaning to buy.
In a full sentence with a subject, Spanish normally uses a conjugated verb.
So:
If you said El chico comprar..., that would be ungrammatical.
Un billete means a ticket, while el billete means the ticket.
Here, un introduces the ticket as something unspecified or not previously mentioned.
So the sentence is talking about a ticket, not a specific one already known to the listener.
Because de ida y vuelta is a fixed expression meaning return ticket or round-trip ticket.
Literally, it is:
So un billete de ida y vuelta means a return ticket.
Spanish uses de here as part of the expression linked to billete.
On their own:
In travel language:
So this is very common vocabulary for transport.
Yes. Spanish often omits subject pronouns, and sometimes even noun subjects if the context is clear.
But in your sentence, El chico is included to make it explicit who is buying the ticket.
Spanish often expresses the same idea with a phrase rather than a single adjective.
English says:
Spanish commonly says:
So instead of one compact modifier, Spanish uses a descriptive expression: ticket for going and coming back.
Not always. Chico can mean:
In a simple learner sentence, boy is the most straightforward translation. But in real life, the exact English word may change depending on age and tone.
Usually in this sentence, no. Billete here is specifically ticket.
A pass in English often means something different, such as:
Spanish might use other words depending on the context, such as abono for a travel pass. So here, ticket is the best choice for billete.