For verbs with irregular humble/honorific forms, are the regular forms still used?


  * There are verbs with _irregular_ humble forms, e.g. the humble form of 借りる is 拝借する.

For these verbs, are the "normal humble conjugations" still used, or
considered grammatical?  
Would お借りする be acceptable?

  * Similarly, there are verbs with _irregular_ honorific forms, e.g. the honorific form of 食べる is 召し上がる.

For these verbs, are the "normal honorific conjugations" still used, or
considered grammatical?  
Would お食べになる and お食べです be acceptable?




“Are they used?” and “Are they acceptable?” are different questions. The
regular forms such as お借りします and お食べになります are used, but they are less formal
than the irregular forms such as 拝借します and 召し上がります. Whether the less formal
expressions are acceptable or not depends on how formally you want to speak.

By the way, you are confusing grammatical terms “respectful form,” “humble
form,” “polite form,” and “honorific form.” 拝借します is the polite form of the
humble form of 借りる, where the humble form of 借りる is 拝借する. 召し上がります is the
polite form of the respectful form of 食べる, where the respectful form of 食べる is
召し上がる. Whether an expression is in polite form or not is orthogonal to whether
it is in respectful or humble form. Honorifics are the broader concept which
includes all of respectful form, humble form, and polite form. For details,
see [Wikipedia](http://en.wikipedia.org/wiki/Honorific_speech_in_Japanese).




“Are they used?” and “Are they acceptable?” are different questions. The
regular forms such as お借りします and お食べになります are used, but they are less formal
than the irregular forms such as 拝借します and 召し上がります. Whether the less formal
expressions are acceptable or not depends on how formally you want to speak.

By the way, you are confusing grammatical terms “respectful form,” “humble
form,” “polite form,” and “honorific form.” 拝借します is the polite form of the
humble form of 借りる, where the humble form of 借りる is 拝借する. 召し上がります is the
polite form of the respectful form of 食べる, where the respectful form of 食べる is
召し上がる. Whether an expression is in polite form or not is orthogonal to whether
it is in respectful or humble form. Honorifics are the broader concept which
includes all of respectful form, humble form, and polite form. For details,
see [Wikipedia](http://en.wikipedia.org/wiki/Honorific_speech_in_Japanese).

