Καὶ ἐμαρτύρησεν Ἰωάννης λέγων
Be able to identify and explain all these parts of εμαρτυρησεν
ὅτι τεθέαμαι τὸ πνεῦμα καταβαῖνον ὡς περιστερὰν ἐξ οὐρανοῦ καὶ ἔμεινεν ἐπ᾽ αὐτόν.
οτι often introduces direct discourse in which case it is translated with quotation marks (know the difference between direct and indirect discourse).
τεθεαμαι is deponent. Notice the reduplication. ?
καταβαινον is a participle modifying πνευμα. It is anarthrous but still adjectival. Here it is an attributive participle which means you will translate it with a relative clause. Here is the evolution (BBG 27.3):
καταβαιν- compound verb stem
καταβαινo add the connecting vowel
καταβαινοντ add the participle morpheme
καταβαινoν the τ drops off (as per noun rule #8)
καταβαινον add the case ending (there isn’t one in this case)
κἀγὼ οὐκ ᾔδειν αὐτόν, ἀλλ᾽ ὁ πέμψας με βαπτίζειν ἐν ὕδατι ἐκεῖνός μοι εἶπεν·
ηδειν is from οιδα. αλλα is always a strong adversative conjunction.
πεμψας is articular; and therefore, adjectival. Here it is clearly a substantival participle. ?
With what (or in what) were they baptized? ἐν ὕδατι υδατι is dative; cf. MBG 199; n-3c(6b).
εκεινος is in apposition to ο πεμπψας με βαπτιζειν εν υδατι. Notice the agreement in case between πεμψας and εκεινος (principle 38). εκεινος is the subject of ειπεν.
ἐφ᾽ ὃν ἂν ἴδῃς τὸ πνεῦμα καταβαῖνον καὶ μένον ἐπ᾽ αὐτόν, οὗτός ἐστιν ὁ βαπτίζων ἐν πνεύματι ἁγίῳ.
This line is the DO of the verb ειπεν. It is direct discourse, so use quotation marks.
εφ is from επι. ?
The αν causes our translation of ον to change from “whom” to “whomever”.
The lexical form is οραω. ? ? . ? ?
καταβαινον and μενον
καταβαινον and μενον are two attributive participles (even though they are anarthrous). Translate them using -ing words: “…descending and remaining…”.
Note that εστιν is a linking verb. The entire phrase that follows is the PN. βαπτιζων is articular and a substantival participle. Since it is a substantive, we expect it to be performing one of the noun functions. The evolution of καταβαινον is given above; here is μενον (BBG 27.3):
κἀγὼ ἑώρακα καὶ μεμαρτύρηκα ὅτι οὗτός ἐστιν ὁ υἱὸς τοῦ θεοῦ.
ἑώρακα is from ὁράω
μεμαρτυρηκα is much more regular.