ます form for る(u) verbs?


I'm a bit confused about the ます form for る(u), I assumed it's just the _[stem
form]_ +ます。

For the verb 召し上がる, the ます form was 召し上がります. But for いらっしゃる, the ます form was
いらっしゃいます?

What's the rule deciding which ます form is it? Is the stem form of いらっしゃる also
just いらっしゃい?




The simple answer is that いらっしゃる is an irregular verb, so treating it as a
regular verb (which is obviously the right thing to do without other
knowledge!) leads to your totally natural confusion. Your thought that ます-form
is just the [stem form]+ます is correct, it's just that いらっしゃる's stem-form is
irregularly いらっしゃい.

To be more specific, いらっしゃる is one of the five polite "ある special class"
verbs:

> いらっしゃる・いらっしゃいます
>
> おっしゃる・おっしゃいます
>
> くださる・くださいます
>
> ござる・ございます
>
> なさる・なさいます

These all behave in the same way: the る does not turn into り/れ for ます-stem
(aka continuative form, 連用形) / imperative stem (aka 命令形) respectively, but
rather い for both the same. This pattern is seen with/"explains" how we get
ください as an imperative form of くださる.

