I understand that the dictionary definition of the expression いけません is "must not do", and that it can be used in conjunction with say なきゃ in a sentence such as the following:
試験があるから、勉強しなきゃいけません。 I have to study, because there will be an exam.
いけません seems to 'conjugate' in the short form present tense as follows:
毎日、練習しなきゃいけないんです。 I must practice every day.
And this is where I get a little puzzled. It seems like いけません would be a conjugation of いける (which presumably would mean something like "to not be permitted to"), but that doesn't appear to be the case. Is there any logic behind this, or is it just a peculiarity? Or am I missing something obvious?