First of all a joke: Past, present, and future walked into a bar – it was tense.

This joke illustrates a spelling issue – should I have written “…walked in to a bar…”? When do you write in to as one word?

While it is not important when speaking, it makes all the difference in writing as regards to meaning and grammar. If you opt for the wrong spelling, your readers are likely to become confused.

To understand the difference between these words, we need to look at grammar. The one-word form, into, is a preposition. Prepositions are used in front of a noun or a pronoun to show, place (we live in an apartment), time (it rained during the day) or direction (a mouse ran across my path).

Into has several meanings, mainly relating to movement, action, or change:

• Describing movement or action that results in someone or something becoming enclosed, surrounded by, or being in contact with something else: Martin put the wine into the fridge.
• Towards the direction of something: the main road leads into the city centre.
• Expressing a change of state: the peaceful demo turned into a violent confrontation.
• Showing the result of an action: the minister was forced into a public apology.
• About or relating to something: an investigation into the incident is under way.
• Used when dividing numbers: two goes into six three times.
• Used informally to mean actively interested in something: she’s into swimming.

As for in and to, they’re words with many meanings and they can perform several different roles in a sentence (they’re both adverbs and prepositions; in is an adjective and a noun as well). You can also use to with the base form of a verb (she had to leave him).

Here are some examples:

• Mum called us in to supper (adverb in, preposition to)
• He caved in to their demands (phrasal verb cave in, preposition to)
• The whole family pitched in to clean the house (phrasal verb pitch in, infinitive to clean)
• I came in to have a cup of coffee (adverb in, infinitive to have)’

Questions to Ask Yourself about Whether to use “in to” or “into”

1. Is to part of an infinitive verb or is it a preposition?

If yes, always write in and to as separate words:

Ministers stepped in to resolve the crisis. 
(in is part of the phrasal verb step in; to resolve is an infinitive verb form)

I just dropped in to see how you were.
(in is part of the phrasal verb drop in; to see is an infinitive verb form) 

He listened in to our phone call.
(in is part of the phrasal verb listen in; to is a preposition that belongs with the noun phrase our phone call) 

2. Does into appear in conjunction with a verb (or phrasal verb) of movement, action, or change, and is it functioning as a preposition that’s linked in meaning to a noun or pronoun?If yes, always write into as one word:

When I first stepped into the room, I had no idea of its size. 
(step is a simple verb of movement; into is a preposition)

Oh no! I dropped my phone into the bath!
(drop is a simple verb of action; into is a preposition)