In this case the difference is not related to tenses. *Erlernen* and *lernen* are two different verbs.

More often than not *er* can be identified with a (non-separable) prefix that implies to do the things until the goal is reached ("Vorsilbe, die eine zielgerichtete Handlung ausdrückt"). Hence, *erlernen* is to learn something until you master it, while *lernen* might only be the process. 

> [**lernen**][1]: sich lernend mit einer Sache beschäftigen, bis man sie beherrscht


You might wish to know something more [here][2]. E.g. *erschießen* means to shoot somebody dead, till the goal is accomplished (i.e. dies); while *schießen* can be only to shoot. Or *ertrinken* (drown); one "drinks" water till one dies.

My favorite is *erinnern* (to see into yourself until you find the thing you are looking for, till you remember). 


  [1]: http://www.duden.de/rechtschreibung/erlernen
  [2]: http://de.wiktionary.org/wiki/er-