To make different words in a field have different styles or fonts, select the words you want to change while you are in Browse mode. You can then choose either Text Style... from the Edit menu or the items you want from the Font and Style menus—which have been added with HyperCard 2.0.
Choosing any of the standard text styles from the Style menu (other than Plain) toggles the setting for that style in the selected text: if a selected word is already bold, and you choose Bold from the Style menu, the word will be unbolded. Choosing Plain from the Style menu will cancel all of the other style settings within the selected text.