"certain" only before noun ～名詞の前の"certain”～

used to talk about a particular person, things, group of things etc without naming them or describing them exactly.
 The library's only open at certain times of day.
 I promised to be in a certain place by lunchtime.
話し手にはわかっているが何らかの理由ではっきり言わない場合 certain を使う