That is a simply astonishing assertion. For one thing - as @margarethall astutely pointed out - it’s essential when addressing more than one person! Also, I do not believe that the ‘polite singular’ use is no longer valid, as this teacher apparently implied. Just look at people being interviewed on S4C, for example - chi is consistently used, for the obvious reasons.
How extraordinary! 
I would say the opposite, if you could only use one or the other I would definitely go for “chi”, much more useful. Your teacher was just flat out wrong on this one I’m afraid, you need them both.
I’ll add that it was said with a large slice of “its a joke, but I’d learn ti first”
Phew, I feel safe to comment now. I’m slowly learning to keep schtum until someone more knowledgeable has spoken-
Joking apart, I think we need them both. S4C was mentioned; the same goes for Radio Cymru talking to anyone, except perhaps one of their peers or a child.
A first language friend of mine, who is much more distinguished than me, always seems to start the conversation with Shwd ych Chi?
I’m guessing it’s just something you play be ear.
It’s also a very curious attitude to language teaching, though one that seems very prevalent these days, particularly with lesser-known or endangered languages, perhaps: ‘Teach them to get by with as little as possible, and make a virtue of this.’ But this is not the road to fluency or confidence, is it?
I really cannot imagine German classes (for example), where they didn’t teach both du and Sie (and indeed ihr!), or French where they didn’t teach both tu and vous.
I agree with that, and am glad we’ve started doing more of it in Level 2 because I’ve been in contact with a local native speaker here in london and she is using it all the time with me!
To be fair this was a single morning session with prospective Welsh learners wanting to learn about Welsh culture and a bit of language. Most of the participants didn’t even know a single word in Welsh-not even thank you, for example. I think it was more of a “we’re learning ‘sut wyt ti’ today but be aware that ‘sut dych chi’n’ is a different way to say it, which unless you’re going to commit to full time evening classes, I wouldn’t worry about today”.
Rereading my original wording, I think I conveyed what they implied a bit more literally than what was actually said, I’d say the message meant to forgo learning it for the 2h session!
True, although in school classes the decision as to which form to use when is simpler - your peers are all ‘tu’, and adults other than your parents (unless you’re very posh) are all ‘vous’. And in Welsh, having seen kids on Rownd a Rownd ‘chi’ their parents I’d have thought it was more important.
Fortunately, as far as French is concerned, there’s a handy guide I came across on Twitter:
Maybe we should make one of them in Welsh and we could carry it around in our pockets and then whenever we go to speak to someone we could get it out and study it before the conversation begins that way eliminating any confusion or embarrassment (and indeed eliminating most conversations). 
Oh fair enough then 
Ditto, Radio Cymru, and I’d bet it’s in constant use in the Welsh Assembly.
In the past, I think they erred on the cautious side and told learners to always use Sie, and don’t bother with “du” because you will never get to know any Germans well enough to address them in the familiar form (= “your German will never be good enough to get to know people that well in a German speaking way” which was probably true in traditional language classes …).
But the world has changed. The internet has changed everything, for one thing.
And even the Germans are a little more relaxed about this kind of thing, especially among young people.
In the private German class that I go to, which has a very relaxed, but reasonably high standard of German, we use Du and Sie interchangeably, with each other and to the teacher (with her consent), because a) she doesn’t think it’s that important in this context b) it means we get regular practice in both forms. We know enough such that we’d be a little more careful in Germany, of course. But it means we can switch from one form to another with reasonable confidence.
Come to think, that’s pretty much how it is with SSiW.
Love that cartoon / flowchart @RichardBuck . I especially like the “Best to ask”. Pretty well defines the general son-in-law-father-in-law (and mother-in-law) situation, I’d say.
Yeah – it just leaves me wondering “Beth ydy Jacques Chirac yn y Gymraeg?” – which I suspect may not be a very useful question to ask anyone…
How do I link several adjectives in a sentence like “It’s interesting but hard”?
Mae hi’n ddidderol ond anodd
or
Mae hi’n ddidderol ond yn anodd
Is the second (and subsequent) “yn” needed?
I’d have thought: …ond mea’n anodd. Only because it sounds Welsh and quite lyrical
I think, for what that’s worth, that it sounds better to me, or at least I’d say it as “mae’n ddiddorol ond yn anodd”.
Another question (should I just start a thread called “Stephen asks inane questions”?): mutations in preterite sentences.
I’ve just been doing some exercises on DuoLingo, and two examples I had were:
Daeth Sioned yma ddoe
Gaeth Sioned dywydd braf ddoe
Why is Cael mutated to Gaeth but Dod not similarly mutated to Ddaeth? As I understand it, both are mutated when in the negative or in a question:
Ddaeth Sioned yma ddoe?
Gaeth Sioned dywydd braf?
Or am I wrong? I know I shouldn’t worry about these things, but they bug me when I see them and don’t know the reasons.
Because SM is optional in inflected (i.e. with endings) preterite and future affirmative (statement) sentences. You could just as easily have
Caeth Sioned dywydd braf
and
Ddaeth Sioned yma ddoe
Having said that, I think SM is the more common option with cael than with dod - but they’re both perfectly OK.
Is this anything to do with the Mi and Fe you sometimes see at the start of such sentences?
