into, onto and up to - the difference
 
When a verb with in (e.g., hand in, dive in) is followed by to, keep the in and the to separate. For example:

Jump in to see if it's cold.       Hand it in to the police.

Be aware though that some verbs can include the preposition into (e.g., dive in, dive into). These rules apply to verbs with on as well.

Watch the verb turn into as it has two meanings: to move into somewhere and to transform something. For example: To turn into the garage. To turn into stone. To avoid ambiguity, use in to for the move into somewhere meaning. The issue with upto and up to is much easier. The word upto does not exist.
 

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In To and Into

The word into is a preposition. It is written as one word.

Example:

She turned everything she touched into gold.

However, on occasion, the words in and to appear next to each other in a sentence, and writers are unsure whether to use into or in to. This happens when the verb in the sentence includes the word in (e.g., hand in, step in, turn in).

Examples:

Paul wanted to hand the purse in to see if there was a reward.
(In this example, the word to is from to see. When a verb is preceded by to, it is said to be in its infinitive form.)

Paul wanted to hand the purse in to the police.
(In this example, the word to is a preposition in its own right.)

More confusion arises with verbs like drive in, put in and fall in. This is because drive into, put into, and fall into are equally valid alternatives.

Examples:

Put the fruit in the basket.
(put in – okay)

Put the fruit into the basket.
(put into – okay)

Put the fruit in to the basket.

Dive in the water.

Dive into the water.
(alternative to above)

Dive in to the water.

Dive in to test the water.

On To and Onto

The guidelines above apply equally to onto. It is noteworthy, however, that onto can mean on top of. When this causes a problem, use on to.

Examples:

After seeing the sheep, we moved onto the cows.

After seeing the sheep, we moved on to the cows.

Up To and Upto

Finally, the easy one: up to is never written as one word.

Example:

I can afford upto 400 pounds.
(should be up to)

It takes up to four hours to hard boil an ostrich egg.
 
IS IT MAGIC?

Writers should be wary of turn into because it has two meanings.

Example:

The car turned into a garage.
(Was the car transformed into a garage or did the car drive into a garage?)

To avoid ambiguity, it is normal to write:

  • Turn in to to mean drive into, walk into, etc.
  • Turn into to mean transform into.
Examples:

He turned the car in to the cul-de-sac.
(Using into would not be wrong – you have a choice.)

He turned the car into gold.
 

See also:

What are prepositions?
What are verbs?
List of easily confused words