How to use if-else in filter in R

I want to check whether values in a column are bigger than the specific value or not. And if there are bigger values, I should draw a line by using these data according to cars' names.

Here is sample data and code:

df <- mtcars[1:5]

df %>% 
  filter(if (mpg>20.0) mpg=mpg else filter(mgp==0))

I cannot check every value of mpg compares to 20.0. I searched and tried to understand some problems like that, yet I could not apply these solutions. I also tried to apply the "when_case" function, but I failed again.