The use of que as ‘to’

Question

Hi there, 
I’ve seen this use of que a few times but can’t find any explanations for the rules.
For example- Hay tantas cosas que aprender. 
Hay muchísimas cosas que hacer.
I know que can be used as to/than in comparisons but I don’t understand the use of que here.

Accepted Answer

Hola Risa
Interesting question! 
This use of que meaning "to" in English happens with relative clauses using the infinitive, which is precisely this case:

Hay tantas cosas que aprender. 
Hay muchísimas cosas que hacer.

This is as if we were omitting a modal verb:

Hay tantas cosas que [tenemos que] aprender.
There are so many things to learn (that we have to learn)
Hay muchísimas cosas que [debemos] hacer.
There are lots of things to do (that we must do/can do)

Other examples could be:

Este tema da mucho que hablar.
This topic is the source of much talk.
No tengo nada que ponerme hoy para la fiesta.
I have nothing to put on today for the party.
¡Vamos! No hay tiempo que perder.
Come on! There's no time to lose.
Tenemos mucho que leer para el examen de mañana.
We've got lots of things to read for tomorrow's exam.

I hope this clarified your doubt. 
Saludos
Inma

Answer

Hi Inma,Thank you so much for your clarification!I’m wondering how can I distinguish between this use of que and a similar use of de when also followed by an infinitive?For example - Es dificil de entender

Answer

Hola Risa
That other use of "de" translated as "to" is explained in this lesson. I hope it helps.
Saludos