翻译“小麦自古就是滋养人体的重要食物，具有养心、益肾、和血、健脾的功效。”

The wheat has been important to norishing the human body since the ancient times and it has the function of improving heart, kidney, blood and spleen.
since前的主句应该是现在完成时，

Wheat is an essential food for nourishing the body of people from ancient time. It has efficacy on hear, kidney, blood and spleen.