ἀπεκρίθη αὐτοῖς ὁ Ἰωάννης λέγων· ἐγὼ βαπτίζω ἐν ὕδατι· μέσος ὑμῶν ἕστηκεν ὃν ὑμεῖς οὐκ οἴδατε,
υδατι is a third declension noun. From MBG, you can see its code is n3c(6b). This takes you to page 198 where you can see the paradigm.
μέσος ὑμῶν ἕστηκεν ὃν ὑμεῖς οὐκ οἴδατε,
μεσος is functioning here as a preposition with υμων as the OP.
εστηκεν is another μι verb from ιστημι. It is third, singular, and perfect tense. The subject of this verb is not explicit. You can understand it this way: “in the midst of you, [one] stands, whom you…” In verse 27, the participle ερχομενος is modifying this implied subject. You can skip this if you like, but if you are wondering why it is translated as a present tense, you can reference GGBB 579f.
The relative pronoun ον is accusative case because it is the object of the verb οιδατε (BBG 14.10). It is masculine singular because it agrees with the implied subject of εστηκεν (BBG 14.9).
. ? ? ? Every clause has its own subject and verb. ?
οιδα is a strange verb. See BBG page 145.
ὁ ὀπίσω μου ἐρχόμενος,
Note the article before οπισω. This article belongs to ερχομενος which means that it is an articular participle and hence, an adjectival participle. Here it is substantival, in apposition to the implied subject of εστηκεν in verse 26.
οὗ οὐκ εἰμὶ ἐγὼ ἄξιος?
This is a dependent clause. It functions adjectivally and modifies the substantival participle ο ερχομενος. The DMW is ου.
ἵνα λύσω αὐτοῦ τὸν ἱμάντα τοῦ ὑποδήματος.
The usage of ινα in this verse is sometimes called “epexegetical” (GGBB p. 476). This simply means that this clause is giving further information about a noun or adjective in the preceding words. Here, you can see that this clause is explaining the adjective ἄξιος. It is telling us more about his unworthiness.
Is υποδηματος a partitive genitive with ιμαντα (GGBB p. 84)?
The only thing we really have to work with here is the sigma. We know that the tense formative of the future tense is sigma. We recognize the first singular ending. Hence, this could be a first person, future verb. However, the future tense doesn’t work very well in this context. Because of the ινα and principle 27, we are led to believe that this verb is probably subjunctive. We know that the mark of the subjunctive mood is a lengthened connecting vowel. This being the case, we know that this sigma might be the aorist tense formative. Here is how it evolved:
ταῦτα ἐν Βηθανίᾳ ἐγένετο πέραν τοῦ Ἰορδάνου, ὅπου ἦν ὁ Ἰωάννης βαπτίζων.
ταυτα is a demonstrative pronoun. You’ll have to supply a noun for it to modify.
Just like οπισω above, περαν is an adverb used here as a preposition. Ιορδανου is the OP.
ὅπου ἦν ὁ Ἰωάννης βαπτίζων.
οπου is a DMW. ? ? ? ?