Questions & Answers about Serums heb ik er drie nieuwe getest.
Why is this sentence split up instead of just saying "Ik heb drie nieuwe serums getest"?
This is called split topicalization. By putting Serums at the very beginning of the sentence, you make it the topic of conversation, similar to saying "As for serums..." in English. The quantity (drie nieuwe) is left behind in the middle of the sentence.
Why do we need the word er in this sentence?
This is the quantitative er. Whenever you state a number or quantity (like drie) but the noun it counts is separated from it or omitted, Dutch requires you to insert er. It acts a lot like "of them" in English.
Why does nieuwe get an -e ending here?
Even though the noun serums has been moved to the front of the sentence, the adjective nieuwe still has to agree with it. Because serums is plural, the adjective takes the standard -e ending.
Could we put er somewhere else, like right after Serums?
No, er generally cannot be separated from the middle section of the sentence. In sentences with split topicalization, the quantitative er usually comes directly after the finite verb (heb) and the subject (ik).