Take, for example, the words loan and lend. Both loan and lend can be verbs. In this context, lend tends to be more popular, although in some circles the reverse is true. In all circles, however, only loan can be a noun. Good writers never use lend as a noun.
Consider the sentences in the example graphic above. The first use (indicated with red underline) is correct; lend is used as a verb (in its infinitive form). As mentioned previously, the writer may elect to use loan instead of lend. Rewriting that sentence would produce
However, writers have no stylistic choices between lend and loan when using a noun. They must use loan because lend has no noun form. Thus, the second use (indicated with blue underline) is incorrect and should be rewritten as follows: