式(shì) - style, type, pattern; formula; ceremony (bound morpheme) in Mandarin

式 (shì) is the -style suffix. Attach it to a place or a period and you get a design: 中式 Chinese-style, 日式 Japanese-style, 法式 French-style, 新式 new-style, 老式 old-fashioned, 美式咖啡 an Americano.

At the front of a word it names a fixed form instead. 式样 is a design, and in the other direction 方式 is a way of doing something, 形式 is outward form, 公式 is a mathematical formula and 仪式 is a ceremony. The common thread is a pattern laid down in advance.

The trap is that it never stands alone. You cannot say 这个式很好; the word you want is 式样, 方式 or 样子 depending on the sense. Treat 式 as a piece that always needs another character bolted to it, and note that the -style compounds usually take 的 before a noun: 中式的菜.