I am teaching English.
因为teaching在句中是谓语的成分,是teach的现在进行时,所以它是现在分词.

my job is teaching english
因为teaching在句中是表语的成分,动名词短语teaching english做表语,所以它是动名词.

温馨提示：要判断是现在分词还是动名词,主要由它在句中做的成分来定夺.
满意请及时采纳,谢谢