using

[English4everyone]

complete the sentences using the infinitive form of the verbs.
Why is it using? Why is it ing form? What is original form?

[Raymott]

"Complete the sentences using the infinitive form of the verbs."
Why is it using?
It has to be; that's the only grammatical form here, although an alternative (necessitating other changes) would be: "Complete the sentences. Use the infinitive form of the verbs"

Why is it ing form?
Why should it not be? It means: "Using the infinitive form of the verbs, complete the sentences." That's normal English grammar.

What is original form?

The bare infinitive of "using" is "use."