You ask a child to read a letter but not out loud.
If I say "please read quietly", then the child might murmur the letter.
If I just say "Please read it", then the child might read out loud like he is taught at school.
Can we say "Please read it in your mind/in your thought/ in your brain..."?
"In your head" or "silently" would be the idiomatic way to express this.
Alternatively "read it, but not out loud".
For most people, beyond a young age, "read" means "read in your head" and only "read aloud" needs normally to be specified.
No, "read it in your mind/thought/brain" is not idiomatic.
A more idiomatic phrasing is "Please read it silently", which means "without making any sound".
