Learn French: Il fallait travailler dur tous les jours.

Il fallait is the imperfect (past) tense of il faut (it is necessary / one must). Here, it translates to "it was necessary to" or "one had to". Just like in the present tense, the il is impersonal and just means "it", rather than "he".

Why is travailler in the infinitive (unconjugated) form?

Whenever you use an impersonal expression like il fallait, the action verb that follows must be in its infinitive form. You are literally saying "It was necessary to work."

I thought dur meant "hard" like a solid object. Does it also mean working hard?

Yes! Just like in English, dur can be an adjective for a physical object (a hard rock) or an adverb to describe how intensely someone does something. When describing a verb like travailler, it never changes its spelling and always means "hard".

Why does tous les jours translate to "every day" instead of "all the days"?

While it literally means "all the days", tous les jours is the standard French phrase for "every day". Remember that because jours is masculine and plural, you have to use the matching masculine plural word tous.