what is reduced infinitive? how we know it?
A reduced infinitive is 'to' without the verb. (Meanwhile, a bare infinitive is the verb without 'to')

Reduced infinitives:

She asked me to go shopping with her, but I didn't want to. -- The reduced infinitive is 'to [go]'.

I promised to marry her, but I never intended to.-- The reduced infinitive is 'to [marry]'.
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