Translation:He has a very big table standing in his room.
I think such variants are not added because they are considered unnatural in English (as opposed to ‘There is a very big table [standing] in his room’).
If you’re a native English speaker and you speak like this, consider using the Report button.
With him = с ним.
У него means ‘at his possession’ or sometimes ‘at his place’ (if него refers to a living person) or ‘near it’ (if него refers to a non-living object). The first meaning, ‘at his possession’, has no direct correspondences in English, so most phrases with it need re-wording.
If I’m not mistaken, the course creators only add the long forms, while short forms are generated by the Duolingo automatically (and sometimes incorrectly).
It isn't. The automatic contractions allow a lot of stuff. I do not think they let you say "The table's big", but "He's been there" should work, which leads to general interchangeability of has and 's.
I gouess, it works if you assume that people here are native speakers.
In modern English has, when it is the "content" verb, usually behaves just like any other verb (i.e. "I don't have a house"/"Do you have a house?"). You do not contract it. It used to be different as late as 100 years ago, when you could also say "I have not a house"—as if "have" were an auxiliary verb.
I believe "Have you a house?" or "We've not much time" sounds old-fashioned to most English speakers these days—but some people speak like that. And you still have "I haven't the slightest clue"; you can also use that structure specifically for stylisation.
Have is easily contracted when it is an auxiliary verb, including its use in "have got". For example, We have looked everywhere can use we've looked and She has got a cup or She has got to go can use she's got .
Duolingo automatically generates an ’s for every ‘has’. The course contributors added "In his room he has a very big table", and the software automatically generated "In his room he's a very big table" as an alternative. This doesn’t work so well for this sentence, but if you don’t like this behaviour, it’s better to write in the common forum: the contributors of the Russian course have no control over this automatic has > 's conversion.
Duolingo converts ‘has’ to ‘’s’ automatically, there is little the course contributors can do about this.