"学生会主席"常见的翻译有以下两种:
the chairman of Student Union
the president of Student Union (这种表达更常见于美式英语中)
注意:student在此以名词的形式作定语,必须用单数形式。
I used to be the chairman/president of Student Union of our university.我曾担任过学校的学生会主席。
Congratulations on you elected as the chairman/president of Student Union.恭喜你当选为学生会主席。