være på at

listen to the pronunciation of være på at
Датский Язык - Английский Язык
be on to
To figure out; to realize the truth

I was planning it as a surprise, but I think he is on to me.

være at
to be to
være at
be to

I know how dangerous it can be to be with her. - Jeg ved, hvor farligt det kan være at være sammen med hende.

Tom says he knows how difficult it'll be to do that. - Tom siger, at han ved, hvor svært det vil være at gøre det.