There are a few nouns to which we have to add -eren instead of -en:
Note the doubled 'm' in lammeren (see keeping vowels short) and the f that turned into v in kalveren (see hard and soft consonants).
The noun 'het been' gets an extra d:
'Been' has two different meanings: 1. bone and 2. leg. If it refers to a leg, the noun has a regular plural:
'Het blad' is irregular because we let the short vowel in the singular turn into a long one in the plural. The plural's syllables are separated by dashes so that you can see that the letter a is now a single open vowel. The regular spelling rules dictate that the d be doubled to keep the vowel short.
||the sheets (of paper)
Both plurals are irregular. The regular plural form would have been bladden.
Latin nouns that end in -um get -s in the plural. Some nouns can also take -a instead of s. The -a ending comes in the place of -um.
||de museums / musea
||de stadiums / stadia
||de podiums / podia
||the (theater) stages
||de aquariums / aquaria
The same can happen to words that end in -on, a less common ending in Dutch.
||de stadions / stadia