英语翻译When you don't know _____ _____ _____ a word,you'd better _____ it _____ _____ a dictionary.

英语翻译
When you don't know _____ _____ _____ a word,you'd better _____ it _____ _____ a dictionary.

the meaning of look up in

when you don't know how to use a word, you'd better look it up in a dictionary.