'In' is being inside a 3 dimensional space, while 'into' is a pentrating action, where you move into that space. A small example to demonstrate the difference between 'in' and 'into'. He is in the room. He goes into the room. This is the main difference between the 2 ,when you don't know whether to use the one or the other. However there are other abstract ways to use both as stated above by Dmitry and Fofa as in 'into music' , 'turn something into something else' , ' in advance' etc. Here it is predetermined which to use and it is usually in the form of expression modes or phrasal verbs.

i'll give you examples for both of them: 1- the paper is in the top drawer. 2- he lived in spain for 15 years. 3- he swam in the sea. 4- in the winter, we use a wood stove. 5- i was born in May, 1995. 6- we finished the project in a week. 7- Tom should be home in an hour. 8- one of the people in the story is a doctor. 9- in the first part of his speech, he talked about the environment. 10- the company is in big trouble. 11-i watched the movie, men in black ,twice. 12- she replied in a quiet voice. 13- i write letters to my friend in English. 14-pupils stand in lines. 15- i work in advertising. 16- put these words in alphabetical order. into 1- how did you get into the house? 2-she always gets herself into trouble. 3-i'd like to go into teaching. 4- i made the dough into a ball. 5- the car had run into a tree. 6- she looked straight into my eyes.

I hope Sami won't confuse if I post here her words: << A rod stroke me (hit? kicked?) into my eyebrow ---> "A rod struck me in my eyebrow." stroke is a noun; it's a disease of the heart, like a heartattack. you mean to use the word struck - the past tense of strike. you could also use hit and/or kicked. also, into isn't used here. hmm, how do i explain? just dont use into with that verb. i would say just use in unless you're using the verb to run - "I ran into the wall. I ran into her." or if you're literally saying inside of something - "I went into the store. I leaped into the pool." That could have some exceptions, though - I might not be thinking of every situation you could fit "into" into (heh, see what i did there? you can also use into in that context. "It may not fit into the story." "You could fit 'into' into this context.") >>