onto
You usually use the preposition onto to say where someone or something falls or is put.
He fell down onto the floor.
Place the bread onto a large piece of clean white cloth.
After many verbs you can use either onto or on with the same meaning.
I fell with a crash onto the road.
He fell on the floor with a thud.
She poured some shampoo onto my hair.
Carlo poured ketchup on the beans.
However, after verbs meaning climb or lift you should use onto, rather than ‘on’.
She climbed up onto his knee.
The little boy was helped onto the piano stool.
If you hold onto something, you put your hand round it or against it in order to avoid falling. After verbs meaning hold, you use onto as a preposition and on as an adverb.
She had to hold onto the edge of the table.
I couldn't put up my umbrella and hold on at the same time.
We were both hanging onto the side of the boat.
He had to hang on to avoid being washed overboard.
Onto is sometimes written as two words on to.
She sank on to a chair.