I agree. I think Mr. Li/ Miss Li/ Mrs. Li would be all OK, but we have a specific word for "professor": "教授"(jiàoshòu)

"Professor Li" worked in the previous exercise; there's no reason it shouldn't work here as well.

I second that.

Duo accepts "teacher Li" now (and always has when I have used it in other exercises).

Definitely. Just report it and they will fix it. Although it takes a while because there are so many reports. I am suddenly getting emails that they now accept answer I suggested three years ago. But quite a bit of them! Must go in batches.

That's what I just put and it said it was perfectly fine.

That IS the answer at the top of this discussion!

The Chinese lessons are new and have to be improved over time with our suggestions via the report button.

高兴 is an adjective, so you don't use 是 with it. You would use 很if it were a positive statement but as it is negative you just replace 很 with 不, so there is no actual verb in the statement because it is just implied.

I wonder as well.

高兴 is a feeling. In chinese, feelings and verbs almost never use '是'.

I think there is a subtle differences between 高兴 and 开心. I read in another post (maybe where kaixin was introduced) that bugaoxing and bukaixin can be two different kind of unhappy.

Not be sure which is which, but one could mean depressed and another angry. Can anyone clarify it, please?

To an untrained ear (including myself) it does sound the same even if a native speaker were to say it to you face to face. What might help is that even though the consonants (the j in jin and t in tin) might sound similar, in Chinese they often say a slight h after the t's like in "tian" ["t(h)ian"]. So, even if "tin" were an actual word in Chinese, you'd know the difference because it would be pronounced "t(h)in" but "jin" is still "jin." Take jian1 间 and tian1 天, they'd sound pretty identical if not for the "h" in "tian"'s pronunciation.

As in you tried "Teacher Li's not happy"? That should work. Report it and they will fix it. (Although it is more normal to say "Mr. Li" or "Ms. Li".)

You're quite right. One of the meanings of "tian" or "天" is heaven. It also means "day". "今天“ or "jintian" means "today". "天天” means "every day".

In British English as spoken in Royal Navy circles “not pleased”, without any reason given for not being pleased, means the same as “not happy”. So you could say “How is the commander today?“ “He’s not pleased“. “About what?“ “Nothing in particular – he’s just not pleased“. But it’s an uncommon use in English I suggest.

We do not usually put "the" if the person's name is there too. It feels like saying "the Charles".

The word order is incorrect in English. In English, you can't put the word "today" after "is." You have to say either, "Today Teacher Li is not happy," or "Teacher Li is not happy today."

I tried it with dictation and it entered 李老师. It's possible you didn't pronounce the tone in 李 correctly, try going lower in tone.

“Today” perhaps.