necessary/ 'nεsɪsrɪ /

1.adjective [+skill] [ quality] [ item] 
(required) 必要的

2.adjective [+consequence] [ connection] 
(inevitable) 必然的

if/when/where necessary
如有必要/必要时/在必要处
I don't want to stay longer than necessary
我不想呆得太久
it may be necessary (for us) to buy a new cooker
(我们)可能应该买一个新的炊具