You are correct. When a noun is used in a sentence or a phrase, you add a "case ending" appropriate to its case. Here the noun "woman" is used in nominative case, therefore "imra'a" becomes "imra'atun". In fully vocalized texts, you'd see a tiny diacritic mark, called dhamma tanween, attached to the end of the word - اِمْرَأةٌ. At the end of a sentence these endings are not pronounced.

And to add to what Yousef said, words ending in ة have a 't' that is normally silent but gets 'activated' when a case ending is added, hence here "2imra2a-t-un". (The 't' is also pronounced in some other contexts, like the 'construct state', but that will come up later I assume). The shape of ة clues you in too: it's like a ه, but with two dots added above to mimic the full 't' letter, ت