What is the difference between IN an INTO

In and Into are other examples of Prepositions of Direction. In and Into are actually interchangeable  when they are used with verbs that show movements.

The boss went in/into his office awhile ago.

However, if the verb does not convey movement, just use "in".

The boss left his briefcase in his office. (not into his office)

Now, if the preposition is the last word of the sentence, you say,

The boss went in. (not into)

And if the preposition comes before an adverb of time, manner or frequency, you also say,

The boss went in twice. (not went into twice)

The boss went in quietly. (not went into quietly)

We can also use "into" as the last word of the sentence.

What kind of situation have you gotten yourself into? (not in)

But, if the question is changed to this form, you say,

What kind of situation are you in? (not into)

In and Into also have unique uses with the verb MOVE. First, you say move in if it is followed by a clause that indicates a purpose or motive.

The army move in for the attack. (not move into)

Second, you say move into if the idea is just to show a simple movement.

The army move in the war zone. (not move into)

In the phrasal verb change into, which means to change someone/something into another thing, "into" there cannot be changed by "in".

In the play, the frog changed into a handsome prince. (not change in)