The formula for the plural form:
noun + s
Examples of plural nouns that end in -s:
When a singular noun ends in a single vowel, you cannot simply add -s to it. After all, a single open vowel is a long vowel. If we add -s to it, it will turn into a (closed) short vowel. In order to keep the vowel long, we add an apostrophe?:
The single e at the end of a word is an exception as it is amute e, not a long vowel. Adding s will not change the mute e.
||the mountain range(s)
||the habit(s), custom(s)
N.B. Correcting for hard and soft consonants is not necessary. An f stays an f and an s stays an s.
Now you probably wonder why we need two different endings for the plural form. We already had -en, which does its job perfectly well. As with many other linguistic phenomena, there is no rationale behind it. It would have been a lot easier if we would just stick to only one ending. But since we have to deal with two, we might as well have a look at the '-s nouns' and the '-en nouns'.