The same rule applies to certain articles (the equivalent of “the”) and indeterminate articles (a class of words containing “a”, “an” and “any”) that are sometimes considered types of adjectiveswww.thoughtco.com/noun-adjective-agreement-3078114. The rule that has no English equivalent is that singular nouns are accompanied by singularadjectives and plural nouns are accompanied by pluralists. Masculine nouns are described or limited by masculine adjectives, and feminine nouns are described or limited by feminine adjectives. Un taco es una preparación mexicana que en su forma estándar consiste en una tortilla que contiene algún alimento dentro. (A taco is a Mexican preparation that, in its standard form, consists of a tortilla containing a little food. Su is a determinator or possessive that changes with number, but not with sex. Estándar is an immutable adjective – the same word would have been used with plural or masculine nouns.) There are a few adjectives known as immutable adjectives that do not change in their form. Most of them are either unusual colors or words of foreign origin. An example is the web as in the página web (the website) and las páginas web (the websites). Sometimes a noun can be used as an immutable adjective, but this practice is much rarer in Spanish than in English.
Spanish students will rarely feel the need to use immutable adjectives, but be aware that they exist so you don`t get confused when you see them. Making a masculine adjective feminine is even easier. Just follow these steps: the “normal” form of adjectives, the form you will find in dictionaries, is singular and masculine.