Learn Dutch: Tijdens de sessie brainstormen we over nieuwe theorieën. - During the session we brainstorm about new theories.

The singular noun is theorie. For nouns ending in -ie where the stress falls on that final syllable (the-o-RIE), the plural is formed by adding -ën. The trema (two dots) on the e ensures you pronounce the extra syllable correctly instead of blending the vowels together into one sound.

How does this plural rule compare to a word like porie (pore) which is also in this lesson?

It is all about where the stress falls. In theorie, the stress is on the final -ie, so we add a full -ën syllable to get theorieën. In a word like porie, the stress is on the first syllable (PO-rie). For unstressed -ie endings, you only add an -n and move the trema onto the existing e of the original word, making it poriën.

Why is the word order brainstormen we rather than we brainstormen?

The sentence begins with the time phrase Tijdens de sessie (During the session). In Dutch, starting a main clause with anything other than the subject triggers inversion. This means the verb (brainstormen) must swap places with the subject (we) to remain in the second position of the sentence.

Are English loanwords like brainstormen conjugated like normal Dutch verbs?