How come when i go to add text to my word doent the text overlaps the existing text insted of just going b4

im in microsoft word and im writing a paper. Im halfway done so i read what i had written. i realized that i needed to add another sentence so i put the cursor in the appropiate place and began to type. i was trying to add a sentence before another sentence but when i started to type the words were erasing the words. do you know what im talking about. like instead of just puting the new sentece before and pushing the other sentence foward it just erased what i had. what can i do to fix this. PLEASE HELP. I NEED TO GO TO BED. ITS 1 IN THE MORNING AND IM TIRED. EASY 10 POINTS THE FIRST ONE TO GIV ME A GOOD ANSWER. THANK YOU