Learn Dutch: Volgens de docent onderzoeken we deze criteria al decennia. - According to the teacher, we have investigated these criteria for decades.

Criteria is the irregular plural of the Latin loanword criterium. In Dutch, many classical words ending in -um get an -a in the plural. While some words allow both an -s and -a ending (like museums and musea), criterium strictly becomes criteria.

Is decennia another irregular plural?

Yes, it is the plural of decennium (a decade). Just like criterium, it is a classical loanword where the -um ending changes to -a for the plural. You will almost always see decennia, though decenniums is technically allowed.

The English translation says "we have investigated", so why does the Dutch sentence use the present tense onderzoeken?

In Dutch, when an action started in the past and is still happening now, you use the present tense combined with a time indicator like al (already). Because the investigation is still ongoing, Dutch says they "already investigate" (onderzoeken ... al) for decades.

Why is the verb placed before the subject (onderzoeken we) in this sentence?