ὅσοι δὲ ἔλαβον αὐτὸν, ἔδωκεν αὐτοῖς ἐξουσίαν τέκνα θεοὺ γενέσθαι, τοῖς πιστεύουσιν εἰς τὸ ὄνομα αυτοῦ,.
ὅσοι δὲ ἔλαβον αὐτὸν is a dependent clause. Every clause has its own subject and verb. ?
δε is an adversative conjunction usually translated “but”.
ἔλαβον is a second aorist verb. Instead of the aorist tense formative, the stem changes. In the present, the stem is λαμβαν-. In the aorist, the stem is λαβ-.
ἔδωκεν αὐτοῖς ἐξουσίαν
αυτοις is the IO (hence, dative case). εξουσιαν is the DO (hence, accusative case).
εδωκεν is from the verb διδωμι which is one of the –μι verbs; there is no need to learn them at this point (see BBG 34.6 if you like). You should already recognize the augment. Recall that verbs ending in -εν are third singular; verbs ending in -αν are third plural. This makes εδωκεν easy to parse.
τέκνα θεοὺ γενέσθαι
This is an infinitive phrase. It is functioning like an adjective modifying the noun εξουσιαν.
τεκνα; whenever you see a noun ending in alpha, think neuter plural; either nominative or accusative (cf. noun rule 1&2). τεκνα is the object of the infinitive γενεσθαι.
γενεσθαι is from γινομαι. It is an infinitive. Infinitives are the easiest to parse. -εσθαι marks second aorist infinitive.
τοῖς πιστεύουσιν εἰς τὸ ὄνομα αυτοῦ,
This is a participle (do you see the participle morpheme; cf. BBG 27.6)? Whenever you see a participle, you should first check to see if it is articular.
If it is articular, then you know it must be either attributive or substantival. An attributive participle is translated just like an attributive adjective; e.g. the running dog, the sweating carpenter.
Remember that even adverbial participles still modify a noun of some sort. In this case, πιστευουσιν is articular, and there is no noun around for it to modify. Hence, it must be substantival. This means it is performing one of the noun functions. You can translate this participle as “believers…” or “to those who believe…” This participle is dative and is in apposition to αυτοις.
Now watch πιστεύουσιν evolve:
πιστεύ- the stem
πιστεύο add the connecting vowel
πιστεύοντ add the participle morpheme
πιστεύουντσι add the case ending
πιστεύουσι the ντ elides (see the footnote in BBG page 248)
πιστεύουσιν movable nu
οἳ οὐκ ἐξ αἱμάτων οὐδὲ ἐκ θελήματος σαρκὸς οὐδὲ ἐκ θελήματος ἀνδρὸς ἀλλ’ ἐκ θεοῦ ἐγεννήθησαν.? If it is a relative pronoun, then it is also a DMW. If a DMW, then a dependent clause is also present. If it is a dependent clause, then it must have its own subject and verb.
The repetition of ουδε here is translated into English by using a correlative conjunction. neither… nor…
αἱμάτων, θελήματος, σαρκὸς, and ἀνδρὸς are all third declension nouns. To check your parsing, use the codes given in the back of MBG.
εγεννηθησαν is a regular form. Know its evolution: