142a1-2 understand ἥκεις or the like in a1 and ἥκω or the like with Ἐπιεικῶς πάλαι in a2 : cf. Crito 43a9-10 ΣΩ. Ἄρτι δὲ ἥκεις ἢ πάλαι; and Protagoras 309b7 ἄρτι ἀπ’ ἐκείνου ἔρχομαι and d5 Καὶ ἄρτι ἄρα ἐκείνῳ συγγεγονὼς ἥκεις. The first line of Phaedrus also has an omitted verb of motion: Ὦ φίλε Φαῖδρε, ποῖ δὴ καὶ πόθεν;
142a3 ἐθαύμαζον ὅτι: verbs of emotion frequently take indirect speech.

142a3 οἷός τ’ ἦ εὑρεῖν: a common idiom. οἷος + τε + εἶναι + infinitive literally means "be such as to (meaning of the infinitive)" which in good English means "be able to (meaning of the infinitive)."
142a4 κατὰ πόλιν: English would have a definite article "in the city": articles are highly idiosyncratic and unpredictable in any language that has them. Smyth §1128 says the article is frequently omitted with prepositions, which suggests both with and without article are normal and there is no difference in meaning. Cf. 142a6 Εἰς λιμένα, 142a7 ἐκ Κορίνθου and ἀπὸ τοῦ στρατοπέδου, 142c6 πρὸ τοῦ θανάτου, d2 εἰς ἡλικίαν, 143a2 κατὰ σχολὴν, and 143a9 ἐξ ἀγροῦ: some of those accidentally correspond well to English usage, while others do not: in one the article is the equivalent of a possessive "his," and many of them are found in other passages with or without the article with no discernible difference in meaning. Articles are among the last thing non-native speakers can accurately predict, because they are so idiomatic and based on a feel for the language rather than rules.

142a8 Ζῶντι ἢ τετελευτηκότι is dative because it agrees with the case of Θεαιτήτῳ in the sentence before it: this interlocking of different character's syntax is a feature of Platonic dialogue, which may create a 'lively' and 'realistic' effect.