れる・られる (Potential)

In addition to 'passive' verbs, れる and られる (the auxiliary verbs) are used when expressing possibility '可能(かのう)'.
られる will be attached to the base form of る-Verbs (the form after る has been removed). Note also that れる and られる themselves behave like standard る-Verbs, so can be conjugated into a variety of different tenses in the same way as a standard verb.

• 明日(あした)までに届(とど)けられる？
  Can you deliver it by tomorrow? (Natural Japanese, and may express 'potential', 'passivity', 'respect', or 'spontaneous occurrence')
• 一人(ひとり)で全部(ぜんぶ)食(た)べられる？
  Can you eat all of this on your own? (Natural Japanese, and may express 'potential', 'passivity', 'respect', or 'spontaneous occurrence')

A common sight in modern Japanese is words where the ら may be removed, 'ラ抜(ぬ)き言葉(ことば)', and れる may be used by itself. However, this is not technically correct Japanese, and may cause confusion in cases where ら is required (with る-Verbs and くる, when られる is expressing 'passivity', 'respect', or 'spontaneous occurrence').

• トムはすぐに新(あたら)しい文法(ぶんぽう)を覚(おぼ)えれる。
  Tom is able to remember new grammar immediately. (Natural Japanese only when expressing 'potential', not 'passivity', 'respect', or 'spontaneous occurrence')
• 彼(かれ)はなんでも食(た)べれる。
  He can eat anything. (Natural Japanese only when expressing 'potential', not 'passivity', 'respect', or 'spontaneous occurrence')

For う-Verbs, they will behave a little differently. Instead of the final kana of the verb changing to an あ sound, and then adding れる (as would be the case with passive verbs), the final kana of the verb will change to an え sound (る will become れ, う will become え, す will become せ, etc.) before adding る.

• この歌(うた)は弾(ひ)けますか？
  Are you able to play this song?
• この池(いけ)では色々(いろいろ)な魚(さかな)が釣(つ)れます。
  You can catch many kinds of fish in this lake.

Interestingly, する verbs do not have a 'potential' form, and the verb できる will be used in its place.

• サリーはほとんどの漢字(かんじ)を書(か)くことができる。
  Sally is able to write most kanji.