Acts 1:13: Why is Ἰάκωβος translated as 'James'?

 At first glance this looks like a long and potentially complicated verse, but it is mostly names.

καὶ ὅτε εἰσῆλθον, εἰς τὸ ὑπερῷον ἀνέβησαν οὗ ἦσαν καταμένοντες, ὅ τε Πέτρος καὶ Ἰωάννης καὶ Ἰάκωβος καὶ Ἀνδρέας, Φίλιππος καὶ Θωμᾶς, Βαρθολομαῖος καὶ Μαθθαῖος, Ἰάκωβος Ἁλφαίου καὶ Σίμων ὁ ζηλωτὴς καὶ Ἰούδας Ἰακώβου

and when they came, 

into the upper part of the house they went up

where they were staying 

Peter and John and James and Andrew,

Philipp and Thomas, Bartholomew and Matthew, 

James of Alphaeus and Simon the zealot

and Judas of James

The use of Ἰάκωβος for 'James' causes perennial confusion.  Here are my comments from the Workbook on the epistle of James:

"Both James and Jacob derive from the same Hebrew name (Yaʿaqov or Yaakov in Roman lettering), but they went through different sound changes on their way to English. For James, one of the key changes was from the Latin ‘Iacobus’ into the later Latin ‘Iacomus’.  ‘Jacob’ [on the other hand] retained the middle ‘k’-sound from the original Hebrew. 

Both ‘James’ and ‘Jacob’ are found in the New Testament, with the latter not declined and almost always referring to the Old Testament patriarch. Here are both names as examples, in the genitive: 

καὶ Μαρία ἡ τοῦ Ἰακώβου καὶ Ἰωσὴφ μήτηρ 

and Mary the mother of James and Joseph (Matthew 27:56)

                    - 

καὶ βασιλεύσει ἐπὶ τὸν οἶκον Ἰακὼβ εἰς τοὺς αἰῶνας 

and he will reign over the house of Jacob forever (Luke 1:33)"

Note that Ἰάκωβος is declined, and we see both the nominative (Ἰάκωβος) and the genitive (Ἰακώβου) in Acts 1:13.

What about the genitive forms Ἁλφαίου and Ἰακώβου?  The genitive here is more or less the equivalent of 'son of'.

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The first verb is εἰσῆλθον, which should be familiar as the 3-P, aorist active indicative of εἰσέρχομαι, 'I enter, I go in.'  This is one of the many derivatives of ἔρχομαι.

Then we have ἀνέβησαν, another 3-P aorist active indicative, of ἀναβαίνω, 'I go up'.

Some other vocabulary:

τὸ ὑπερῷον, -ου:  'upper room, upper story, upper part of a house'

κατανέμω:  'I stay (with), remain, dwell'; the form καταμένοντες (masculine plural nominative, present active participle) is a hapax legomenon, the only use of any form of this verb in the NT.