أَتَجَمْهَرُ في فَمِي
مُعَلَّقًا بينَ اللُّغةِ والعالمِ
We can see that بين is in the accusative case. Why is this?
We discussed in a previous post the accusative case and when it’s used: to give additional information to a grammatically complete sentence. (Check out the previous post for a more in-depth explanation.) The word بين is a ظرف مكان (adverb of place) and means “between”. You can remove it and all words attached to it and the sentence would still be complete. The verse translates to “I muster crowds in my mouth / suspended between language and the world.”You can say, “I muster crowds in my mouth, suspended.” Because of this, بين is in the accusative case, as adverbs of place usually are.
Adverbs of place often come in the form of a مضاف (first word in an idhaafa construction). This is because they’re grammatically incomplete without an additional noun. بين means “between”, so the sentence has to include the things that are talked about: between what? You can’t say, “I muster crowds in my mouth, suspended between.” This sentence would be incomplete.
Note that the idhaafa construction is often taught to non-native speakers as a possessive construction, but while it’s used as the equivalent of the possessive in English, it has many other uses. The idhaafa is simply a syntactical structure of two nouns attached to each other, where the second noun is essential to complete the sentence.
Also note that بَيْن and other adverbs of place are nouns, and they can indeed be used as regular nouns. The word البَيْن means “the division/disunity”. For example, you can say, ازداد البين بيننا, which translates to, “The division has increased between us.”
The word اللغة is the مضاف إليه (second noun in an idhaafa construction) and is therefore in the genitive case. In Arabic, the genitive case is called مجرور. This translates to “something that is pulled”. The first noun in an idhaafa construction is called جار, i.e. “puller”. The puller has to pull another word along with it to complete its meaning. Think of it like a horse and cart: you can’t make a cart move unless you pull a horse along with you for the ride. Because of this, the second word in an idhaafa is in the genitive case. This case signifies a very close relationship of a word with the word that precedes it. The word that precedes it cannot do without it.
The word العالم is also in the genitive case. Even though it’s not structurally attached to بين, the و here is واو العطف and as such makes it take the same case as a previous word. (See this post for a more in-depth explanation of واو العطف.)