Verbs are marked for one of three aspects:
intentional, progressive, and perfective.
They occupy a role similar to tenses in English,
but focus not on the time the action happens
but on what stage the action is at in terms of completion.

The intentional aspect indicates that an action
is planned, but not yet started.
The progressive aspect indicates that it
is in progress, but not yet completed.
And the perfective aspect indicates that
the action has been completed.

There are two words corresponding to "not".
Their meanings depend somewhat on the aspect of the verb.

With the intentional and progressive aspects,
the first one indicates that the action is not being done,
"I'm not doing something,"
while the second indicates that one is trying not to do it,
"I was trying not to do something."

With the perfective aspect,
the first one indicates that an action is not yet completed,
"I haven't done it yet,"
while the second indicates that it was never completed,
"I never managed to do it."