Aquel vs. aquél
Note: The unaccented form "aquel" can function as a pronoun (like "aquél") if it can be unambiguously deduced as such from context.
Aquel
-
that (masculine; far from speaker and listener)
Aquel invierno hizo mucho frío. (It was very cold that winter.)No conozco a aquel señor. (I do not know that gentleman.)
Aquél
-
that one (masculine; far from speaker and listener)
Este coche es más grande que aquél. (This car is bigger than that one.)Aquél fue un día horrible. (That was an awful day.)