1) Onto can be used as a positional word to show someone or something moving upwards or forwards.
Eg:
The man climbed onto the roof to save the cat that got stuck chasing a bird.
The driver sounded the whistle and moved the train onto the track .
2) It can also be used to show an awareness to something eg:
The detective followed the man down the alleyway, the man stopped and turned...the detective knew the man was onto him so he continued walking straight past him to throw him off the scent.
Answers & Comments
1) Onto can be used as a positional word to show someone or something moving upwards or forwards.
Eg:
The man climbed onto the roof to save the cat that got stuck chasing a bird.
The driver sounded the whistle and moved the train onto the track .
2) It can also be used to show an awareness to something eg:
The detective followed the man down the alleyway, the man stopped and turned...the detective knew the man was onto him so he continued walking straight past him to throw him off the scent.
You can be onto someone’s lies.