Bueno, al salir de la tienda tienen que caminar...

This phrase comes from the Travelogue 9.3. I've seen examples in the course like this where it is "al" followed by the verb in the infinitive and others where it is "el" followed by the same. I tried to find examples of the latter without success (which probably would have answered my question as to why if I could have seen them side by side).

I found another example of "Al...": 6.3) Al regresar a la costa para abordar el barco...
 

¡Hola Steven-W15!

Al is a combination of a + el, and it's often used before an infinitive in the sense of "on" or "when" followed by a verb ending in "-ing" in English. For instance, al salir de la tienda would be more closely translated as "on leaving the store" and al regresar a la costa would be like "on returning to the coast."

The definite article el can also appear in front of infinitives, but the effect here is to essentially turn the verb into a noun. In English, we would add an "-ing" to the end of the verb to achieve this same effect. For example, el leer makes the verb leer "to read" act like a noun, and would be the same as saying "reading" in English. El aprender would be "learning." 

I hope that this answers your question! Do let me know if anything is still unclear.

Hasta la próxima,

Liss

Many thanks, Liss - the explanation is very clear. I'll be keeping an eye out on "el + verb" examples and will get back to you if I have any questions.
 

¡De nada Steven-W15! :) Do indeed let me know if you find any that you have questions about!

Ah, that's a perfect example! I'm glad the explanation still makes sense. :) ¡De nada!