"It was the worst night of my life."
Translation:Det var den värsta natten i mitt liv.
I can't get the ad blocker to go away on that site.
But I found another site: https://www.omsvenska.se/ord-och-uttryck/ordval/samre-eller-varre
It says that "sämre" is the most common comparative for "dålig". It is used when something is not as good as something else.
And it says that "värre" is the most common comparative for "illa". It is used to compare with something that is already bad and can be thought of as "even worse".
Duolingo accepts "sämsta" here, but by putting up "Another correct solution", it implies that "värsta" is better. Maybe this sentence is comparing bad nights, not just nights in general.
1) When can you say "i livet" instead of "i mitt liv"? I'm thinking of "Den vackraste stunden i livet var den när du kom" - the lyrics of an actual song. 2) is starting this sentence with Den instead of Det actually wrong or poor form? "Den var den värsta natten i livet" was marked wrong, trying to figure out why.
You can generally use the definite instead of the possessive whenever it's obvious to which life you're referring. Where to draw that line can be muddy, but it doesn't work in this sentence since removing the possessive means you could be talking about anybody's life. In your lyrics example, it can probably be safely assumed that you mean your own life.
Swedish always defaults to the neuter option when the subject isn't known, grammatically speaking. This tendency is so strong that most natives will usually use det är even when the subject actually is known.