Where you've got a plural amount of things, but there's only one of each item, we tend to use "there is" rather than "there are":
In the above article, note the section on single items being counted separately:
There is a ball. There is a box. There is a ball and a ball.
Except, of course, when referring to the BBC. I suspect this issue is related to another singular plural question which varies between U.S. and U.K. English: referring to entities such as companies or teams. The U.L. tends to use the plural, "Virgin Atlantic are planning a new route", whereas in the U.S., the proper usage is singular, "United Airlines is planning a new route."
In my opinion (which seems rather in question of late...)
- There is a ball and a box on the grass
- There are a ball and a box on the grass
- A ball and a box are on the grass
I'm currently fighting over whether I'd accept: - A ball and a box is on the grass (everything I know screams "no", but it still feels oddly okay)...
My only worry with the current translation "A ball and a box are on the grass", is that it comes up in the lesson called "there is", which tends to suggest the translations should be "there is / there are" sentences.
But as an all round, catch all, grammatically sound, non-controversial sentence "A ball and a box are on the grass" is probably your best bet.
Even though I still fight for "There is a ball and a box".
On my own.
In the corner.
Generally "on" the grass, unless it's really long / tall grass, in which case "in"; probably at least partially obscured by the grass.
To the original question "ball and box is" would be absolutely incorrect in the U.S., and apparently of some debate in the U.K.
This question, though would be more important for the Russian-->English course than this course.
There difference here is that this sentence is formed around "There is...".
Where "there is" pops up, the singular/plural divide goes a bit funky for some reason, in some dialects.
Where the two things are the subject of a standard sentence, like in your example above, I completely agree that "is" would be weird and inappropriate.