As the resident native English speaker at my office, I regularly have conversations like this one:
“Hey, Nikki…?”
“Yes…?”
“Do I say ‘lit?’ Or is it ‘litten?’”
“Um. Excuse me?”
“You know. Is ‘lit’ an irregular verb?”
“What are you trying to say? What’s the sentence you want to form?”
“No. I just want to know if it’s irregular.”
“It’s not litten. It’s lit.”
Though I have, as almost all American students have at one point or another in their high school careers, studied grammar–and currently think myself to be an excellent (though not perfect) user of it–I no longer recall exactly which verbs are irregular and which are regular. Do I use verbs correctly? Of course! But can I list for you even five irregular verbs in the English language without consulting my trusty friend Google or beginning to conjugate a lengthy list of verbs in my mind? No.
Because English is my mother tongue (Oh no! Starting a sentence with ‘because’ is a huge grammatical no-no! But don’t worry, I’m finishing the sentence with an independent clause. Ahh! Starting a sentence with ‘but’ is a definite no-no! But it’s okay, seeing as how this is an informal blog, and none of you are here to actually read about English grammar.), these rules are no longer rules to me. I live and abide by them. They are burned into the depths of my mind. Sort of like the rule that says you’re not supposed to steal. I don’t think about it. I just don’t steal. So now my knowledge of the ins and outs of English is being tested and I must say… Mrs. Smith probably wouldn’t be very proud.
And I’m only telling you about the verbal interrogations I receive.
On the flip-side, I look forward to the days when I will be asking for the past participle form of some seldom-used French verb to my friends. Revenge is best served verbally, no? 
As with most things you write about, I can so relate to this! Unfortunately my knowledge of English grammar exists only in the “I don’t know why but that’s just the way it is” realm. Nico and I once had a discussion for twenty minutes about the difference between “this” and “that” and when you should use them. So much fun to explore our own language from another’s perspective!
So true! I always loved English class in school, and never had a problem learning and understanding the rules of grammar. But now that I’ve been out of practice for explaining the “hows” and “whys” for homework exercises, I find myself a bit challenged when asked really specific questions. So I guess, in a sense, I’m not just learning French over here, but English too!