¨A¨ only for horses and feet??

Hola Seth,

There isn’t a single perfect rule here, but there is a useful pattern.

We usually use “a” with forms of movement that involve your body directly or riding an animal:

a pie, a caballo

These are more like fixed expressions, especially a pie and a caballo, which you’ll hear all the time.

For most other types of transport, we use “en”, because you’re traveling in or on a vehicle:

en tren, en coche, en autobús, en bicicleta

The idea of being “inside” the vehicle can help, but it's not a complete rule. For example, we still say en bicicleta, even though you’re not enclosed.

Also, with some animals, both forms are possible:

a camello and en camello

In those cases, en is often more natural in modern usage.

So the best way to think about it is:

• “en” is the default for transport
• “a pie” and “a caballo” are fixed expressions you should remember

Everything else can vary a bit depending on context and speaker.

It’s not completely arbitrary, but it’s not perfectly logical either, so exposure helps a lot.

I hope this helps.

Saludos

Inma