"My father is next to my mother."
You should use よこ because となり is used for non equals elements Like using となり makes the mother and the father not equals. At the opposite, よこ makes the father and mother equals : you wont use it to say "My mother is next to the table" I am not sure at all but you should look in the way (I learned this rule on this forum Duolingo it is not from me)
I saw someone else posted a good answer for this on another exercise.
となり is for two SIMILAR objects that are next to each other. For example two houses, two people standing next to each other etc.
よこ is for two DISSIMILAR objects next to each other. For example a person standing next to a house.
A way to memorize this is by the famous Japanese city of よこはま, which I think someone translated to "next to the beach" (despite google translate not recognizing はま... idk I'm not that good at 日本語). The CITY is next to the BEACH, these are DISSIMILAR objects, and therefore the town is called よこはま and not となりはま, if that makes sense :)
However, by this "rule" I was surprised the answer for this exercise isn't using となり, but instead よこ (mom and dad are both humans). Don't know what's up with that, so someone more experienced please fill in :)
As far as I'm aware both are fine, they just mean slightly different things
父は - speaking of my father, he's next to my mother
父が - it's my father who's next to my mother
so one is generally talking about where yr dad is, the other is sort of specifically pointing him out, or it's focused more on the location and then saying who's there. It's subtle (and I think learning the nuances of は and が is like a life quest)
"Father is mother's next to," was how I tried to think of it, and bam, it was actually correct. Interesting how Japanese treats locational descriptions such as right, left, above, beneath, in front of, behind, and next to as nouns possessed by people or objects. If you start re-wiring your brain to think of it in that way, it starts to help.
The inconsistency between は and が in Duolingo use is driving me insane. I have just completed other 3 sentences in the same exercise structured identically like this one where が was correct and は incorrect in this function, and now all of a sudden it is the other way around?
I get it that sometimes both are valid (with only a slightly different nuance), and sometimes only either of them, but I get the sneaking suspicion that Duolingo doesn't know which either.
Okay, so Duo's absolutely terrible at pointing this out because while it follows the Japanese rules it never really explains nuance or certain omissions. は is almost always the topical marker, not of every sentence, or even in its entirety. Because it can be used as a phrasal or notion/action topic marker as well (look up examples such as のは, ことは etc). It's very difficult to contextually determine whether a topic is omitted in text, which is common practice, but you'd have more than a single sentence to go on. が on the other hand is only selectively used as a topic marker. When it is, the general rule to look out for is whether the information following it is new to the listener (added to this idea, it is info that couldn't be readily deduced by the listener either for whatever reason). Other times it's the verbal subject indicator (like を) as a kind of secondary は. And even further, it serves as a tone softening/humble particle at the end of 'intent' sentences (I'd like to...). This should be sufficient to go very far with, but there are a few other nuances, enjoy the journey.
It has to be "haha no yoko ni" because it needs to specify that よこ refers to the mother. To do that you use の, which can specify possession: XのY = X's Y, the Y of X. The に moves to stay with with よこ because it has to directly follow the word/phrase it refers to, 母のよこに = at mother's side, beside mom.
います refers to people and animals and あります is used for things; those two you use to describe what people and things are doing or where they are. They're the "is" in "that is here" or "he is running" (それはここにあります; 私は走っています). です is the "is" in "that is red" or "she is tall" (それは赤いです; 彼女は背が高いです). It doesn't make any distinctions between people and things. You can use it for descriptions, including anything about the person or thing like titles or names or descriptions. If you wanted to say, "I am your big brother" you'd use です (俺はあなたの兄です) If you used that to explain why you were on the top bunk, you'd use います (だから上のベッドにいます) If you needed to point out that your bags were already up there anyway, you'd use あります (俺の荷物はもう上にあります)
は and が are a tricky beast; they don't work in that way. But to put it in most basic terms, imagine that someone is asking you a question before you respond: は: "What's your father doing?" "My father is (standing) next to my mother." が: "Who is standing next to your mother?" "My father is."
Ignore for a moment that "being" isn't an action; I'm just trying to demonstrate how both can be used but it changes what part of the sentence you're putting focus on: the subject or whatever the subject might be up to.
In English we have rules for this as well: we omit context when the question is asked: What's your dad doing? He's working. Who's working? My dad is.