Hi, The main difference is that 'into' is a preposition by itself. It carries the meaning of going from one place to another, or of changing from one state to another. Using your example sentence, we can say: 'They want to break into the office.' Here it's implied that they are outside the office and want to break in, from one place to another. 'The caterpillar changed into a butterfly.' In this sentence it is not about location but rather state. The caterpillar is changing from the state of being a caterpillar into a new state: being a butterfly. Now, let's look at your second sentence: 'They want to break in to the office.' Here the 'in' and the 'to' are actually separate. The 'in' is a part of the phrasal verb 'break in' and the 'to' gives a sense of direction, pointing to the office. It can also be possible that the 'to' is part of a verb. For example: 'He saw a bar and walked in to have a drink.' Here the 'in' is a preposition modifying 'walked' and the 'to' is part of the infinitive, 'to have.' How do we know when to use each one? A simple way to do it is to replace 'into' with its opposite, 'out of.' If it works, this is a suitable place to use 'into.' Again, looking at the first sentence: They want to break into the office. -----> They want to break out of the office. This sentence is okay. Now, looking at another example: He saw a bar and walked in to have a drink. -----> *He saw a bar and walked out of have a drink. The sentence above containing 'out of' is wrong, therefore we can't use 'into' here. Hope this helps. Have a nice day : )