Agent / doer (fa'il)
Every verb needs a doer. In Arabic that doer is marked in raf' (usually a damma), and it normally follows its verb. It may be a visible noun, or a pronoun hidden inside the verb.
Don't confuse it with نائب فاعل (the deputy agent): the fa'il does the action; the naa'ib stands in for a dropped doer in the passive.
Can the doer be hidden?
Yes — in قُمْ (“stand”) the doer “you” is a hidden pronoun inside the verb. It is still the fa'il.
Why does the Quran sometimes use a feminine verb with a masculine plural noun?
Non-rational plurals are obligatorily treated as feminine singular (جَلَسَتِ الْجِمَالُ), and rational broken plurals or collectives may optionally take the feminine form — قَالَتْ رُسُلُهُمْ (Ibrahim 14:10). Both are standard rules of the fa'il, not irregularities.
Does the verb change when the doer is dual or plural?
No. With an external noun doer the verb stays singular — نَصَرَ الْمُسْلِمُونَ. Only the gender of the doer changes the form; the dual and plural verb forms belong to pronoun doers.