Learn Dutch: Zijn voeten zijn groot.

In Dutch, zijn has two completely different meanings that both happen to be used here. The first zijn is the possessive pronoun "his". The second zijn is the plural verb "are" (from the infinitive verb zijn, meaning "to be").

Because "voeten" is plural, shouldn't "groot" have an -e at the end?

No, because groot comes after the verb. In Dutch, adjectives only get an -e ending when they are placed directly before the noun they describe (like in the phrase zijn grote voeten). When the adjective comes after the verb to describe the state of the noun, it stays in its basic form.

Is the plural of "voet" irregular like the English "feet"?

Not at all! While English changes the vowel (foot to feet), Dutch uses a completely regular plural here. You simply add -en to voet to get voeten. The oe is considered a single vowel sound, so you don't have to drop a letter in the plural like you would with a double aa or oo.