“This one's supposed to be good.”
You're at a video store, looking for a DVD to rent for the weekend. You see a movie that you read a positive review of. You want to convince your wife to watch it, so you say this.
This one's supposed to be good.
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"Supposed to" can be used to express facts or predictions that other people have made and you have heard. A weather forecast is a good example. The newscaster has predicted what the weather will be like. You have heard this prediction, so you repeat it to another person as "it's supposed to ___":
Another example is renting a movie that's gotten good reviews:
"Supposed to" also has other meanings in other situations, like when you're telling someone what they should be doing:
You're supposed to call if you're going to be late.