When should „To jest” be used instead of „To” by itself? This sentence accepts both „To jest długa ulica” and ”To długa ulica”, and I don't understand why. Other sentences I've had „To jest” marked wrong, so I feel like I'm guessing in the dark.
When you mean "This/That/It is" you should use "To (jest)". "Jest" can be omitted and the sentence remains correct.
I guess the ones that had "to jest" marked wrong were probably questions of X is Y pattern. They will work as X to Y, or X jest Y (instrumental for Y here).
Here, as mihxal has said, "to" actually is separate and means this/that/it.