how do I differentiate between the masculine and feminine in nouns?

the feminine version of nouns will always end in an "a" sound such as tawila or nafatha. there are also exceptions to the rule. for example pairs of objects and environmental aspects are also feminine despite the absence of the "a" sound such as shams or qamr. notice the absent of the "a" but it is still despite that, feminine. The feminine version of nouns are also used according to who is being spoken to. for example, a male engineer is a muhandis, while a female engineer is muhandisa.