Short sentences make us think clearly. Making sentences shorter helps us think clearly about English. We need to understand what is the main idea and what is extra. Making sentences shorter helps us practice grammar. We have to take sentences apart carefully, and carefully adjust them to fit the pieces back together again.

It looks like this:
It’s good practice to look at any sentence you write and try to make it shorter.​  (17)

It’s good to practice making your sentences shorter.  (9)

Practice making your sentences shorter. (5)

Make shorter sentences. (3)

1,2,3,4
1 The sample sentence is 18 words long.
2 Write it again in 12 words.
3 Write it again in 7 words.
4 Write it again in 3 words.

Because
Shortening  sentences usually makes the meaning clearer. It also means we have to recognize the parts of the sentence. What is necessary and what is extra. Shortening sentences also requires a good command of grammar.  We need to recognize word groups and recognize when words have to change as they take on new roles in a shorter sentence.