意義相同.在句子中的用法有些差異.
if it is necessary /if necessary /是假設(shè)條件,未必是現(xiàn)實(shí)情況.多用于條件狀語(yǔ)從句.
if necessary 多用于句末,相當(dāng)于if it is necessary 中省略了it is
when it is necessary 可用于句首或者句末,是條件狀語(yǔ)從句,說(shuō)明的是現(xiàn)實(shí)情況.那 We can start the work next week ______ 是不是都可以用？都可以用，沒(méi)問(wèn)題。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