Questions & Answers about رابطهی آنها با همدیگر عالی بود.
Is there any difference between using همدیگر (hamdigar) and یکدیگر (yekdigar) in this sentence?
Both mean 'each other' and you could easily swap همدیگر for یکدیگر here. همدیگر is generally more common in everyday spoken Farsi, while یکدیگر can sound slightly more formal or written, but both are perfectly correct.
Why do we need the preposition با (bâ) before همدیگر?
Just like in English, the pronoun 'each other' needs a preposition to show how the people are interacting. با means 'with', so با همدیگر translates exactly to 'with each other'.
Why does رابطه (râbete) have a ی added to the end of it?
This is the ezâfe connecting the noun رابطه (relationship) to its owner, آنها (they). When a word ends in a silent 'heh' (ه), like رابطه, the ezâfe is written as a ی so it is pronounced as 'râbete-ye' (the relationship of).
Since آنها (they) is plural, why is the verb بود (was) singular instead of plural?
The verb always matches the main subject of the sentence. Here, the subject is رابطهی آنها (their relationship). Even though it belongs to multiple people, the 'relationship' itself is a single thing, so it takes the 3rd person singular verb بود.