“I think the filter needs to be changed.”
You have a swimming pool in your yard. The water looks dirty, so you say something to your husband about it.
I think the filter needs to be changed.
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Use "___ needs to be ___" to talk about work that someone needs to do:
It needs to be done soon.
English speakers use "be __" to avoid saying exactly who should do something. For example, you might say this this to an employee:
The files in this folder all need to be reviewed.
Instead of this:
You need to review the files in this folder.
The first one sounds softer and less direct.
Or you can say "___ needs to be ___" if you don't know who should be responsible:
That old building needs to be knocked down.
Use "___ doesn't need to be ___" when it's OK not to do something:
No, they don't need to be cooked. You can eat them just like that.
A lot of machines use "filters".
For example, an air conditioners has a filter to clean dust out of the air. A car engine has a filter to clean dirt out of the oil. Water purifiers have a filter that cleans the water.
After you use a filter for a while, it gets dirty and stops working as well. When that happens, someone has to "change" the filter. That means replacing the old one with a new one.