I have to report that Duolingo doesn't see 'Ты знаешь' as a good beginning of this sentence.
It does. Or at least it does now
Would "ты знаешь мосты в Петербурге?" not be considered a valid response?
That's "the bridges in Saint Petersburg" (в + prepositional). The exercise says "of Saint Petersburg" (genitive).
what about the word Saint????
Not necessary in everyday Russian
"Ты знаешь мосты в Санкт Петербурга" was originally given as the correct answer. Now Duo will only accept "вы знаете мосты в Петербурга." ????
It's weird that it was given as the correct answer. As far as I know, в Петербурге means "in Saint Petersburg". The exercise says "of Saint Petersburg", I assumed it required the genitive (without в).
I accidentally wrote ' Петроград ' . Is that used any more ?
As Leningrad it's not used any more.
Same goes for the has-been Stalingrad (now Wolgograd). And also for Kaliningrad that was Königsberg once.
Those days are dead and gone
Знаешь мосты Петербурга not accepted.
is there a difference between writing peterburg and saint petersburg?
Is there a particular reason why duo insists I add "вы" when it's implied in the verb?
It is usually there in native speakers' speech. We can accept the answers without them, of course (omitting the pronoun is not technically incorrect).
When I studied in Moscow (granted it was years ago) the professors generally left it out at least for the вы form, maybe it was a local thing. It did count my answer as wrong without the pronoun though.
Couldn't one write "Петербурга мосты" or "Петербургские мосты"?
петербургские/петербуржские мосты are, in principle, possible. Your first option, Петербурга мосты is not something you are likely to hear outside poetry.
Are the bridges of St Petersburg as nice as the bridges of Madison County?
My good friend is from St. Petersburg, so I asked her this question. Apparently there are a thousand bridges in the city