Write an expression for the calculation divide 10 in half and subtract from 50 divide into fifths

There are several issues with this English description, as there often are.

We assume you mean you want to divide 10 into two equal parts, then subtract one of those parts from 50.

... (10/2) is half of 10

When that is subtracted from 50, the result is

... 50 - (10/2)

We assume you want one fifth of this result. Other interpretations are that you are subtracting 10/2 from (50/5), or that you are using the difference as the denominator with 1/5 or some other number of fifths as the numerator (dividing <the difference> into 1/5).

Then your expression might be ...

... (1/5)(50 -10/2)

There are ways to write math expressions in English so they are unambiguous. This isn't one of them. It must be done very carefully, as English is often intentionally vague and subject to a variety of interpretations (as most human languages are). Good punctuation helps.