Acts 1:14

οὗτοι πάντες ἦσαν προσκαρτεροῦντες ὁμοθυμαδὸν τῇ προσευχῇ σὺν γυναιξὶν καὶ Μαριὰμ τῇ μητρὶ τοῦ Ἰησοῦ καὶ σὺν τοῖς ἀδελφοῖς αὐτοῦ

these all were persisting with one mind (in) the prayer with women and Mary the mother of Jesus and with his brothers

 

There are two common third-declension nouns in this verse, of different sub-groups.  Remember that listings of third declension nouns often include the genitive singular form, which points to the correct spelling for the entire declension.

1   ἡ γυνή, γύναικός -  woman

In this verse, γυνή is seen in the dative plural, with a -ξ- in the ending.  

2  ἡ μήτηρ, μητρός - mother

The full declension of both of these feminine nouns can be found on the 'Declension Tables' page, here.

In other vocabulary, ὁμοθυμαδὸν is an adverb, meaning something like 'of one mind, unanimously'.  Of the 11 uses of this adverb in the New Testament, 10 are in Acts.

Μαριάμ is an indeclinable version of the name translated 'Mary' in English; it is Hebrew in origin.  The other version in the NT is ἡ Μαρία, which can be declined.

This seems to be the last mention of Mary (the mother of Jesus) in the New Testament.

Now, what about the verbs?

Προσκαρτεροῦντες is a masculine plural nominative form of the present active participle of προσκαρτερέω, 'I persist, persevere (in)'.

The form ἦσαν προσκαρτεροῦντες might be considered periphrastic; i.e., the use of the 3-P imperfect form of εἰμί plus a present participle, to convey a meaning similar to the imperfect: 'they were persisting, steadfastly continuing'.