I love this one …
Or, when a husband, not learning Cymraeg shouts “hoffi coffi” and nicely servs it . 
Or when @brigitte says “OK, Tatjana, now we should put some German into our language mix…” and you start to laugh loud because you all of a sudden can’t remember anything but Cymraeg no matter how hard you’re trying to say something in German.
and, yes, “ll” I automatically pronouce it Cymraeg way.
When you’re trying to speak to a family member in pig-Latin but keep trying to use Cymraeg, which makes said family member look at you as if you’ve grown a second head!
That’s my favourite story from this forum:)
Or when your English-speaking boyfriend, who was initially quite ironic about you learning Welsh, learns to say “Bore da, cariad, sut wyt ti” and learns the words “broga” and “llyfant” and all the other absolutely necessary words in Welsh:)
Or when your friend’s child (Russian) learns to say Welsh numbers and place names before he learns them in English at school!
Yes, I can be persuasive.
What is pig-Latin, might I ask?
I believe latin which is not too good??? Just guessing too.
See: https://en.wikipedia.org/wiki/Pig_Latin
Sometimes popular with children who think their parents won’t understand what they’re saying…
Oh, thank you very much:) I remember as a child I used to do it too.
We use it in my family when we want to talk about birthday/Christmas presents in front of the kids…my young cousins haven’t quite worked it out yet. When they do, I might just have to teach my aunt Welsh instead!
This might be a bit of an odd one, but I remember the faint surprise when I realised one day that hearing a soft mutation instinctively “felt feminine” in situations like y ddynes, y gath, but also
somehow “felt masculine” in stuff like ei ben, ei gar - and my brain apparently considered that to be perfectly natural (rather than, say, super weird and confusing). It was a strange moment
I like this one!
Which means, of course, that I can know say to people “I know of a child who spoke Russian and Welsh, but no English…”
Well, he will eventually learn English at school, but I think it’s a sort of a poetic justice that he learned to speak some Welsh first:)
And I love this one …
… when you translate the clues in a crossword instead of trying to solve them.
Love this. And I really love the feeling in the mouth of the first person possessive: Mae hi yn fy mhoced, or whatever. There is a point when a lot of the mutations click into ‘feeling right’ or ‘sounding right’, which, for me, gives one a very personal connection to the language.
I quite like the mutations for first person possessive too. For example I use Fy nghariad quite often as opposed to girlfriend, even in english and then have to correct myself when people look at me weird.
If you change it to ‘fy nghwraig’, don’t make the same mistake I did and say ‘fy ddraig’ instead … or ‘fy nghwaith’ (that didn’t go down well at all). I tried to explain that we were encouraged to make mistakes… 
Haha, I suppose it depend of the temperament of your partner.
It is such a tempting mistake, though - er, for many people, although obviously not you, and in fact clearly not me either, better keep myself out of this…
Oh, do parthers not take as cute expression being called “fy ddraig”? - hehe. I surely do!