Grammar fix
let on
reveal a secret (often used in the negative: not let on)
to allow other people to know something that is secret or private, often by accident or by not hiding it well; to admit or reveal information that you were supposed to keep hidden.
Hear it in context
Notice where the phrase sounds natural.
She didn't let on that she already knew about the surprise party.
In my opinion, it's important not to let on if you know a friend's secret, because trust is essential in any relationship.
Fix the pattern
One small grammar move makes it sound right.
Not: Don't let on him about the party.
Say: Don't let on (to him) about the party.
'Let on (to someone) ABOUT/THAT' — don't put the person straight after 'let on'.
Which sounds natural?
Tap the one a native speaker would say.
'Let on' naturally means reveal you know something (usually in the negative). 'Gave no indication' is formal.
Build natural chunks
Words that usually travel with it.
let on
- didn't let on
- never let on
- that she knew
- about it