"Mom does not have him now."
Translation:У мамы его сейчас нет.
Если честно, по-моему это предложение звучит нескладно в любом виде :|
It's difficult to understand what this sentence means without a context.
Here's an example of this sentence in a context:
— У мамы же есть личный стилист? 'Mom has a personal stylist, doesn't she?'
— У мамы его сейчас нет. 'She doesn't have one right now.'
I'm not sure why, but У мамы нет его сейчас definitely sounds much less natural.
Russian nouns (words naming people, thing and phenomena) have several forms called cases.
The dictionaries list nouns in the default case called 'nominative'. The nominative of 'Mom' is «ма́ма». We use it when the noun does an action (Ма́ма чита́ет 'Mum reads'), or in 'X is Y' sentences (Ма́ма — писа́тельница 'Mum is a writer', both ма́ма and писа́тельница are nominative forms).
However, after the preposition «у» you need to use a different case, genitive. Genitive of «ма́ма» is «ма́мы». We also use genitive with «нет» to express absence of something, so «его нет» means 'there is no him/it' (it's often a way to say that someone is away).
The preposition «у» is used to indicate possessor. Originally, it meant something like 'near', but when we use it about people, it means 'at someone's possession, at someone's place'.
Literally, «У ма́мы его сейча́с нет» means 'At Mum's [possession/place], him/it now there-is-no'. This phrase can be used to say Mom doesn't have someone or something, or when someone or something is not in Mum's place.
You can’t move «у» from «мамы», «у мамы» is an indivisible unit. Otherwise, everything here can be moved to emphasise some word. The general trend is that known information comes first, new information comes last (unless you emphasise new information with the intonation). Most of those word orders would sound pretty strange out-of-context, though.
E.g. if you put «нет» first, you’ll get a sentence about absence. The listener knows something or someone is absent, and you clarify what/who is absent. This is rarely something anyone would say.
Duo didn't say it was an incorrect statement. Google translate didn't say it was a correct translation.
Duo said it wasn't a good translation because they felt it didn't convey the meaning of the example regardless of whether your answer was grammatically correct.
Google makes no judgement about the value of the sentence. It simply makes a relatively poor effort to convey the meaning of each word in the sentence.
Duo wants the answer to reflect a logical flow of importance in the order of the words used. Google doesn't care at all as long as the sentence makes some sort of sense.
Your answer says that the most important thing in the sentence is what is happening right now. You follow that up with indicating mom's place is not where he is.
The Duo example says that the most important thing about his location relates to his mother following that up with the fact that he, at the present moment, is not there in that order.