Acts 1: 12

Τότε ὑπέστρεψαν εἰς Ἱερουσαλὴμ ἀπὸ ὄρους τοῦ καλουμένου Ἐλαιῶνος, ὅ ἐστιν ἐγγὺς Ἱερουσαλὴμ σαββάτου ἔχον ὁδόν

then they returned to Jerusalem from (the) hill called 'of the olive grove', which is near to Jerusalem of a Sabbath having a journey 

A word about the proper noun Ἱερουσαλὴμ. It has Hebrew and Aramaic underpinnings, as one would expect, and has been transliterated into Greek.

There are two spellings of common use in the NT: Ἱερουσαλὴμ, a feminine, indeclinable noun, and Ἱεροσόλυμα, which is feminine in the singular, but neuter in the plural.  Perhaps the feminine gender comes from the Greek word for city, πόλις, which is feminine, although I am not sure.  

Plural?  There is some discussion about the plural forms (see here, for example), and I have no definite answer.  I will note that the -μα ending typically denotes a third-declension neuter noun (πνεῦμα, σῶμα, ὄνομα, βρῶμα), so there may have been a tug of war between Jerusalem being a city (πόλη, feminine) and the -μα ending of the transliteration.

At any rate . . .

The main verb in the verse is ὑπέστρεψαν: 3-P, aorist active indicative, ὑποστρέφω, 'I turn back, I return'.

The subject is an understood 'they', again referring to the apostles from previous verses.  A few English translations add the word 'apostles' to make this clear, but it isn't in the Greek.

So we have

τότε ὑπέστρεψαν εἰς Ἱερουσαλὴμ 
then they returned to Jerusalem

and the rest of the verse explains where they were returning from, and how far away this place was from Jerusalem.  To whit:

ἀπὸ ὄρους τοῦ καλουμένου Ἐλαιῶνος

        from a hill / mountain the one called of the olive grove 

Ἐλαιῶνος is the genitive singular form of the third-declension masculine noun Ἐλαιών, 'a place of olives', i.e., an olive orchard.  This is typically translated as 'Mount of Olives', or 'Mount Olivet'.

The next phrase is clear enough:

ὅ ἐστιν ἐγγὺς Ἱερουσαλὴμ 

the one (i.e., the hill or mountain) near Jerusalem

The following phrase, however, is not clear:

σαββάτου ἔχον ὁδόν

Literally, 'of Sabbath having a road', which does not make sense in English.  The  idiom, much discussed, seems to be 'a Sabbath's journey away', meaning the distance that one could travel on a Sabbath.