Questions & Answers about Mascara's heb ik er twee zwarte.
Why do we need the word er in this sentence?
This is the quantitative er. Because the noun (Mascara's) has been moved to the front of the sentence (split topicalization), it is separated from its quantity (twee). Whenever you state a quantity of something but the noun isn't right next to the number, Dutch requires you to use er.
Why is Mascara's placed at the very beginning instead of saying Ik heb twee zwarte mascara's?
Placing the noun at the front makes it the topic of the sentence, similar to saying 'As for mascaras, I have two black ones' in English. Because Mascara's takes the first position, the verb (heb) must take the second position (V2 rule), pushing the subject (ik) to the third position.
Why does zwarte have an -e ending here?
Even though the noun is sitting at the beginning of the sentence, zwarte is still logically modifying the plural noun Mascara's. Adjectives modifying plural nouns get an -e ending. Notice how English uses a dummy pronoun ('two black ones') to hold the place of the noun, but Dutch simply leaves the inflected adjective hanging on its own.
Just as a spelling refresher, why does Mascara's get an apostrophe-s instead of just an -s?