Thank you for your kind answer. TranVanHaiNam answered my question. As you may already know, in English, the two expressions are 'equivalent' but there are times when I want to use the verb 'to be' to emphasize the adjective. For example,
His pants are short (implying that the pants shrunk OR that he outgrew them or some other abnormal situation...) In those cases, I would not say 'His short pants...'