Which article - LO or EL? 1

Lo — the neuter article for abstract concepts — Lo + adjective or past participle creates an abstract noun meaning "the … thing/part/aspect": lo importante es aprender (the important thing is to learn), lo mejor de Madrid (the best thing about Madrid). There is no noun after lo in this construction.

Lo + adjective/adverb expresses degree — Lo also combines with adjectives/adverbs + que to express how much or how: No sabes lo difícil que fue (You don't know how hard it was). ¡Lo bien que canta! (How well she sings!). This construction is very common in Spanish exclamations.

El vs lo — the deciding question — Use el when a specific concrete masculine noun follows: el libro, el mejor jugador. Use lo when the concept is abstract and no masculine noun follows: lo mejor (the best thing), lo dicho (what was said). No noun can follow lo in this usage.