The difference between "on" and "onto"

Question

I still can't figure out when to use on and onto, e.g., "He put the bag on/onto the table". -Reader

Answer

The difference between on and onto

The prepositions on and onto can be used in many of the same sentences, which makes them confusing. However, their meanings are not exactly the same, and they are not always interchangeable.

Onto has the word to in it, which reminds us that its meaning includes the sense of movement towards something. The preposition on does not have this sense of movement, and it tells you only about location. 

This difference is hard to see in the examples you have asked about, because the verb, put, itself, suggests moving something from one place to another, and the object, bag, ends up in the same place in both sentences.

1.He put the bag on the table.

2.He put the bag onto the table. 

The difference is easier to grasp in these two sentences:

1.Louie drove his golf cart on the sidewalk. 

2.Louie drove his golf cart onto the sidewalk. 

The meanings of these two sentences are very different. Sentence 1 tells us only where Louie was driving. Sentence 2 tells us that Louie started somewhere else (probably the street or the driveway), and moved in the direction of the sidewalk, where he ended up. 

Guidelines to follow 

Happily, there are guidelines that you can use to determine which of these two prepositions is best. In most sentences that are about fixed situations, without movement, on is correct and onto would be considered wrong.  

• The book is lying on the table. 
• How long can you stand on one foot? 

In most sentences that are about something that moved itself, on and onto are both correct:

• We climbed on/onto the roof of the building.
• The cowboy leaped on/onto this horse.

In most sentences that are about moving something from one location to another, onto is nearly always correct and on is sometimes correct:

• Transfer the data onto a disk. 
• Slide the cake onto a plate. 
• He put the bag on/onto the table. 

I hope this helps.