句子成分划分有助于理解结构和内容——
【The man主语】【 with glasses介词短语,定语】【 is谓语】【 my math teacher.表语】
=【The man主语】【 who is with glasses定语从句】【 is 主句谓语】【 my math teacher.表语】
=【The man主语】【 is 主句谓语】【 my math teacher.表语】and he wears glasses.
可见,使用is,是因为,句子的主语是单数对象【The man】.【 with glasses】是介词短语 ,作主语的定语.
祝你开心如意!