[*] 193. A Periphrastic form, as the name indicates, is a “roundabout way of speaking.” In the widest sense, all verb-phrases consisting of participles and sum are Periphrastic Forms. The Present Participle is, however, rarely so used, and the Perfect Participle with sum is included in the regular conjugation ( amātus sum , eram , etc.). Hence the term Periphrastic Conjugation is usually restricted to verb-phrases consisting of the Future Active Participle or the Gerundive with sum .
[*] Note.--The Future Passive Infinitive, as amātum īrī , formed from the infinitive passive of eō, go, used impersonally with the supine in -um, may also be classed as a periphrastic form (§ 203. a).