"Er-" is a verb modifier that modifies the underlying verb. Even though it is not a preposition, it performs a similar role to prepositions such as an- auf- aus- that modify verbs when they are prefixes before a verb.
"Er-" is a modifier that means to "complete" or "fulfill." So lernen is the underlying verb for "to learn," and "erlernen" modifies it to "completely learn" (or "master.")
In any event, erlernen is not a "tense" of lernen; instead, erlernt is the past tense of erlernen. Gelernt is the past tense of lernen.