You go out to a restaurant with your friend and don't want to order much because you're saving money. You say this to explain that you're saving because you want to buy a house.
We're trying to save up so that we can afford a house.
This means to save money and build it for a certain purpose, like buying a house or car.
The structure "___ so that ___" explains your reasons for doing something.
I'm trying to be quiet so that the baby doesn't wake up.
In normal spoken English, you can drop "that":
I'm trying to be quiet so the baby doesn't wake up.
This means to have enough money to buy something:
We can't afford to send our kids to private school.
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