Lesson 17: Enduring the pain of ている and てある
This brings us to ている and てある, which use いる/ある as auxiliary verbs. ている indicates an enduring action/state. For most verbs, this means progressive or continuous (enduring action), but for others, it means the complete state of the action remains (enduring state). The second kind of verb is usually "intransitive", meaning that they cannot normally accept a direct object, like 死ぬ "die".
食べている I'm eating. (enduring action)
死んでいる He's dead. (enduring state)
The い in ている can be dropped. This is normal but not formal.
食べてる I'm eating.
てある is about something being in an enduring state resulting from an action. It's important to remember that てある expresses a state, not an action.
ケーキ作ってあるよ The cake's done! (Someone has made the cake.)
てある takes the direct object and treats it as a subject. This means that the verb's normal "logical object" is usually marked with が or は or unmarked, like the cake in the example above. But because japanese is linguistically weird, sometimes を is used, too. You can think of using を here like taking the entire phrase " <thing>を作る" and conjugating that phrase with てある.
Finally, it expresses a state, not an action, so てある is not "the passive". Japanese has a real passive that we'll cover soon.
Normally, intransitive verbs plus ている act alike transitive verbs plus てある, aside from using the logical subject instead of the logical object. This is what "enduring state" means for ている.
ている only shows up for い-adjectives when the い-adjective is the end of a verb phrase, like 食べないでいる. This is the て "form" of the verb phrase 食べない. You can't do this with い-adjectives on their own.
Something like なくている is nonsensical because the くて form of い-adjectives is always a conjunction, never a way to string words together. Remember not to confuse the words "conjugation" and "conjunction".