i

No matter where the verb is in the sentence, it has to agree with, or change its form, to match its subject (noun or pronoun). Sometimes that means looking back quite a long way in the sentence:
The measurements, which were taken every second, were essential to the experiment.
Essential to the process, the particles must fall in alignment.
The particles’ speeds and the jet flow are measured using a probe.
They were aligned precisely in order to ensure accuracy of fall and flow.