In the previous paragraphs, you have learnt how to distinguish between
long and short vowel words.
In general, long vowel words should stay long, regardless of any changes in the word (e.g. a singular noun turning into a plural, or when you conjugate a verb's infinitive). The same goes for short vowel words.
In the following sections, we use the example of a plural and a singular noun to illustrate how a word maintains its long or short vowel. Of course, this mechanism applies to all kinds of words (verbs, adjectives, adverbs, etc.).