Do we? I thought it was 'use the infinitive'. In all my life, I have never heard someone utter the word, "Put the verbs in the infinitive". I googled and found 9 hits for it, most of those saying, ""Put the verbs in the infinitive form". So….. what? 4 or 5 people agree with you?
It is definitely, "put the verb in the infinitive" rather than "put the verb at the infinitive."