Why does くださる have an irregular infinitive form (ください)?


I may be using the wrong term, but I understand the infinitive form of a verb
in Japanese to be the form we add ～ます to. In the case of ichidan verbs, we
take away the ～る and add ～ます、and for godan verbs we change the ending う-line
to an い-line and add ～ます. Why then, is くださる not conjugated as くださります in
infinitve form, and instead conjugated as くださいます? Additionally, why do we only
use ください most of the time?

EDIT: To clarify, I'm interested both in the functional purpose of this, and
any historical or cultural purpose of this if they exist. Thank you!




It is just because it is easier to pronounce. It is called イ音便.

This also happens in English like "I am" and "I'm."

But, since this change started around 8th century, now it is rather natural to
say in イ音便 form.




It is just because it is easier to pronounce. It is called イ音便.

This also happens in English like "I am" and "I'm."

But, since this change started around 8th century, now it is rather natural to
say in イ音便 form.

