(from) letting out

If not required, it sounds better to the native ear. However, either sentence is a little awkward.

Yes, although you could use the word "keep" instead of "stop", as well.

We had to stop her from killing herself.

We had to stop them from dancing in the streets.

Yes, it's correct.

What do you mean by the above sentence?

We tried in vain to stop Eileen (from) letting out the secret.

Do you mean 'from' should be used?

Thanks.

Yes, I believe "from" is necessary.

Thanks.

I agree with Mame. The "from" is necessary unless you wish to say "We tried in vain to stop Eileen's letting out the secret.

In your sentence "letting" is a gerund (verb form used as noun) so you can not use the noun "Eileen" as the subject of the noun "letting". In my example "Eileen's" is a possessive adjective for the noun "letting". In Mame's "letting" is the object of the preposition "from".

Thanks.