意义相同.在句子中的用法有些差异.
if it is necessary /if necessary /是假设条件,未必是现实情况.多用于条件状语从句.
if necessary 多用于句末,相当于if it is necessary 中省略了it is
when it is necessary 可用于句首或者句末,是条件状语从句,说明的是现实情况.那 We can start the work next week ______ 是不是都可以用？都可以用，没问题。1、we can start the work next week if it is necessary2、ifit is necessary，We can start the work next week 3、we can start the work eekif necessary 4、when it is necessary We can start the work next week5、we can start the work next weekwhen it is necessary