There are three important rules to remember here:

Neuter gender

Along with masculine and feminine, Latin also has a neuter gender meaning “neither,” referring to how it is neither masculine nor feminine. Thus neuter gender is often applied to things which don’t have a natural gender, words like: “war” bellum, “iron” ferrum, or “danger” periculum. But it’s not as simple as that. There are many exceptions to this rule and thus in Latin things which are masculine in gender are not necessarily always male in nature. The same holds true for the other two genders.

Substantives are words that are fundamentally adjectives but they function as nouns. Such as the adjective “good.” It can serve as a substantive, that is, as a noun. So, for instance, if you talk about the “goods” in the store, “goods” is a noun meaning “the good things” in the store. The adjective is serving as a noun and that’s a substantive. Similarly, you can talk about a “swift,” a fast-flying bird. Birds are nouns but the word “swift” is at heart an adjective. In English we often show that an adjective is functioning as a substantive by pluralizing it or putting an article in front of it, such as, “electronics,” or “the young and the restless.” But Latin adjectives contain more information encoded into them than their English counterparts which makes them much more naturally easy to convert into substantives because, for instance, they have gender and gender naturally implies a gendered substance. Hence the principle that Latin substantives derive their substance from their gender. In other words, because a Latin adjective is masculine, it implies that the substance or the unstated noun lying behind the adjective is “man,” or “men” if the adjective is plural. If an adjective is feminine in gender, it implies “woman,” or if it’s plural, “women,” and if it’s neuter, “thing” or “things.” For an adjective to serve as a substantive it must have no noun to go with in its sentence. Otherwise, there’s no need to evoke substance from its gender. In other words, if bonus has liber to go with it, it’s “a good book.” There’s no need to add “man” to “good” if you’ve got “book” there.

So this is how substantives work: you’re reading along in a Latin sentence and you come to an adjective. It doesn’t have a noun to modify. You look at its gender. If the gender is masculine you add “man” or “men” to the translation of the adjective. If the adjective is feminine, you add “woman” or “women,” and if it’s neuter, “thing” or “things.”

Here are some examples of substantives: