Questions & Answers about Не будь грубым с гостями.
Why is грубым in the instrumental case instead of the nominative (грубый)?
When telling someone how to act using the imperative будь (be), Russian usually puts the following adjective in the instrumental case. It highlights a temporary behavior rather than a permanent trait.
Why does Russian use с (with) here instead of a word for "to"?
English says "rude to", but Russian expresses this specific type of behavior towards someone as being "rude with" them. This is why you use с followed by the instrumental case.
Why does the plural noun гостями end in -ями?
It is the instrumental plural of гость (guest), triggered by the preposition с. Because гость is a masculine noun ending in a soft sign (-ь), its plural instrumental ending becomes -ями instead of the standard -ами.
Can I use the short form adjective груб instead of грубым?
Yes, не будь груб is completely correct and has the same meaning. However, using the full adjective in the instrumental case (грубым) is very common and often sounds a bit more natural in everyday spoken Russian.