Periphrasis is exactly what it sounds 
            like it is. Peri, the Greek preposition 
            περί
            meaning around (with the 
            accusative) and phrasis, 
φράσις
            , a phrase, are put together to refer to a 
            round about way of phrasing something, or we might say, a sort of 
            circumlocution. In fact, that is what the Greek compound 
            περίφρασις 
            meant. We especially use the term with reference to a construction using two 
            words, usually a form of 
        εἰμί and a 
            participle, to express one verbal idea. We do this in English when we say something like, 
            "I will be arriving in the morning."
            The tense expressed by a periphrastic 
            construction may be the equivalent of something that could be 
            expressed more simply by a single word. Turner asked, "What possible 
            distinction can there be between 
            ἐβάπτιζεν 
            and ἦν βαπτίζων 
            in Jn 322.23...?" (Syntax, p. 87).On the other 
            hand, there is no single verbal form that could express the meaning 
            of the future perfect periphrastic expressions  
            ἔσται δεδεμένον 
            and ἔσται λελυμένον 
            (shall have been bound, shall have been loosed; Mt. 
            16:19).
            With that in mind, understand that 
            when we talk about a periphrastic tense, we are not 
            necessarily referring to a distinct verbal idea, but rather to a 
            particular periphrastic means of conveying an idea.
            
            Present Periphrastic Tense
            The present periphrastic tense is formed by a 
            present tense form of
            
        εἰμί and a present 
            participle. An example of present periphrasis is the phrase
            ἐστιν μεθερμηνευόμενον 
            = is translated. This phrase occurs a half dozen times in the 
            NT, including the following instance at John 1:41:
            
            ε
ὑρήκαμεν 
            τὸν Μεσσίαν ὅ 
            ἐστιν μεθερμηνευόμενον Χριστός
            We have found the Messiah, which 
            is translated, 'Christ' 
            
            Imperfect Periphrastic Tense
            The combination of an 
            imperfect copulative and a present participle yields a meaning very 
            similar to if not identical with the simple imperfect tense. 
            Consider Mk. 1:22:
            
            
            ἦν γὰρ
            διδάσκων αὐτοὺς ὡς ἐξουσίαν ἔχων καὶ οὐχ ὡς οἱ γραμματεῖς.
            
            
            
            
            For he was teaching them as having 
            authority and not as the scribes.
            The imperfect has linear 
            Aktionsart in its own right, but Hewett suggests that periphrastics 
            may "enhance the continuing nature of the activity" (p. 151). 
            Perhaps that is true in some instances, but in our present example, 
            is there a great difference between 
            ἦν...διδάσκων 
            in verse 22 and ἐδίδασκεν 
            in verse 21?
            Perfect Periphrastic Tense
            This consists of a 
            present tense form of 
        εἰμί 
            and a perfect participle, as in Jn. 6:45: 
            
            ἔστιν γεγραμμένον ἐν 
            τοῖς προφήταις
            
It is 
            having been written in the prophets
            or more simply in English,
            It is written in the prophets
            
            Future Periphrastic Tense
            This may be expressed by 
            a future form of 
        εἰμί and a 
            present participle, as in Lu. 21:24:
            
            Ἰερουσαλὴμ ἔσται πατουμένη 
            ὑπὸ ἐθνῶν
            
Jerusalem
            will be trampled by Gentiles
            Future Perfect Periphrastic 
            Tense
            We mentioned one of the 
            few examples of this construction earlier. Actually, there are two 
            occurrences in one verse at Mt. 16:19. A future form of 
        εἰμί and a 
            perfect participle combine to express the idea "will be having 
            been..." i.e., in the future, something will already have been an 
            accomplished fact.
            
            καὶ ὃ ἐὰν δήσῃς ἐπὶ τῆς γῆς 
            ἔσται δεδεμένον ἐν τοῖς οὐρανοῖς
            and whatever you bind on the 
            earth will be having been bound in 
            heaven
            or more simply in English,
            and whatever you bind on the earth will 
            have been bound in heaven
            
            καὶ ὃ ἐὰν λύσῃς ἐπὶ τῆς γῆς ἔσται λελυμένον ἐν 
            τοῖς οὐρανοῖς
            and whatever you loose on the earth
            will be having been loosed in heaven
            or more simply in English,
            and whatever you loose on the earth will 
            have been loosed in heaven
            
            There are other 
            periphrastic constructions, but the foregoing introduction will 
            suffice for now. Actually, you don't need to memorize these so much 
            as understand them. And you will be able to understand other 
            periphrastic constructions as you encounter them because they 
            usually mean what you would expect them to mean simply by 
            considering the component parts.