Question

for toning up / a good / facial muscles / it / exercise / is make the sentence​

Answer 1

it is a good exercise for toning up facial muscles

Answer 2

Answer:

It is a good exercise for toning up facial muscles: 'it' signifies the use of a pronoun to describe something which is a form of exercise.

Only one exercise is described and hence, 'is' is used in the singular form.

Later on in the statement, it is described what the exercise is doing, and that is it is toning up the facial muscles.