Stressing a word when speaking, by saying it longer, louder or with higher intonation, will show the listener which word is key.
By stressing the correct word, you make it easier for the listener to grasp the meaning of your sentence. However, if you put emphasis on the wrong word, then you will change the meaning completely.
Consider the following sentence, "I didn't steal your wallet." Say it to yourself. You would probably stress the negative, "didn't" and that would be correct.
However, if you want the same sentence to mean something different, you can stress a different word.
- I didn't steal your wallet, someone else did!
- I didn't steal your wallet!
- I didn't steal your wallet, I just borrowed it!
- I didn't steal your wallet, I stole his wallet!
- I didn't steal your wallet, I stole your cell phone!
Some learners might think stressing words is risky—you don't want to make a mistake, so you might play it safe by not stressing any word. This is not a good strategy because flat speech sounds robotic and boring. Not only that, but it's confusing for the listener. Native English speakers expect some words to be stressed, so not getting that feels very strange.
Studying stress is important. It can be useful to read about it because you can choose your pace and read it slowly. Think about how something would sound, and try saying it out loud. If you would like to hear it, then ask your next instructor to read it.
So, as you listen to English and practice speaking, pay close attention to which word is stressed in the sentence.
play it safe [set phrase]—choose to do the safest option.