ing形式的语法疑问
问题描述：
ing形式的语法疑问
句子是这样的
their intensities must be within the working limits of the materials composing the surfaces.
materials composing the surfaces 这个帮我分析下,ing放后面表示什么,如何理解,是宾补吗?请说得详细些,
must be within为什么这中间要加个be啊?
答
of the materials （WHICH) composing the surfaces
composing the surfaces做materials的定语,解释说明materials.
within是介词而非动词,而must后需be动词或实义动词,介词前用be动词.