The phrase (not sentence) 'long-standing army' is grammatically correct. It means 'an army that has been established for a long time.'
As the longstanding matriach of the family, it will be lonely since she will be away for the holidays.
there is a longstanding rivalry between USC and Clemson football teams.
As an adjective it is one word "longstanding" (traditional, done for many years).
an army :)
"How are you" is a correct sentence.
Yes, this sentence is correct.
The word "longstanding" is an adjective.
A longstanding custom is called a Tradition
You are palpitating. This is a correct sentence.
With the exception of not capitalizing the 'y' at the start of the sentence, it is a correct sentence.
That sentence is grammatically correct.
no it is not a correct sentence.