Grammar Point of the Week： You can't do that それはいけない

Maybe this isn't quite a grammar point, but I've started watching the new show featuring Eita and Ueno Juri, called Sunao ni Narenakute - Hard to Say I Love You. Ueno's character, Haru, has a catchphrase kind of going: それはいけない。 She's referring to Nakaji's (Eita) flattery/kindness to her, which can be mistaken by her female sensibilities as fancy rather than politeness. When she saysそれはいけない, it's kind of like “You can't do that. You can't lead a girl on like that." This seems like an easily-adapted-to-the-situation, useful phrase. I imagine you can use it to mean "that's a no go" etc.

I suppose to make this more of a grammar entry, I will talk a bit about potential conjugation, as in "You CAN'T do that", vs regular conjugation/dictionary form ("You (will) do that"). Basically:

For -る verbs: Take of the -るand add -られる. 食べる　＝＞　食べ　＝＞　食べられる

For all other verbs: Take of the -うand add -える.　行く　＝＞　行ける

The irregular するbecomes できる.  来るbecomes 来られる.

From this point, all the verbs are -る verbs. So, if you wanted to change it to the more formal -ます form:

食べられる　＝＞　食べられます　・・・　行ける　＝＞　行けます　

However, I often heard people dropping the -らpart of the -るverb conjugation. I've heard that this may be naturally happening because the potential and causative form of -るverbs are identical, and people naturally want to differentiate them. Thus:

食べられる＝＞食べれる、来られる＝＞来れる

When speaking I think you will mostly hear -れるinstead of -られる; but when writing make sure you use the technically correct form.