“Into” is a preposition which often answers the question, “where?” For example, “Tom and Becky had gone far into the cave before they realized they were lost.” Sometimes the “where” is metaphorical, as in, “He went into the army” or “She went into business.” It can also refer by analogy to time: “The snow lingered on the ground well into April.” In old-fashioned math talk, it could be used to refer to division: “Two into six is three.” In other instances where the words “in” and “to” just happen to find themselves neighbors, they must remain separate words. For instance, “Rachel dived back in to rescue the struggling boy.” Here “to” belongs with “rescue” and means “in order to,” not “where.” (If the phrase had been “dived back into the water,” “into” would be required.)

Try speaking the sentence concerned aloud, pausing distinctly between “in” and “to.” If the result sounds wrong, you probably need “into.”

Then there is the 60s colloquialism which lingers on in which “into” means “deeply interested or involved in”: “Kevin is into baseball cards.” This is derived from usages like “the committee is looking into the fund-raising scandal.” The abbreviated form is not acceptable formal English, but is quite common in informal communications.

See also turn into.