expand 侧重指“上下、左右、前后的面或体的伸展”,如:
Irons expands when it is heated.
铁加热时就膨胀.
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extend可以表示时间和空间的“延长",还可以用于比喻意义上的“延长”,或范围的“扩展”.如：the extended meaning of a word (词汇的引伸意义),to extend (or lengthen) a road (延长道路),to extend (or lengthen) one's stay.(延长某人的逗留)
Can you extend your visit for a few days more?
你能把你的访问再延长几天吗?
The railway will be extended next year.
明年将要延长这条铁路.
…,and the project has now been extended.
……而且此项计划现在已经扩大了.
He is extending his eyes with radar;…
人类借助雷达来扩大自己的视野.