'poposcerunt' is the verb in the 3rd person plural perfect tense, from 'posco'.
'stantes' is a participle in the present tense from the verb 'sto' with a nominative plural ending, so we know it is agreeing with the 'sacerdotes'.
'pro' is a preposition and it takes the ablative case, here 'templo'.
That leaves us with 'silentium' which could either be nominative or accusative, since it is a neuter noun, but as we already have a noun in the nominative, it must be accusative.
Does that help?