The barest possible event is laid out here, and each element is described individually in a later section. The overall structure is similar to that of a CK2 event, with some tweaks to the syntax and a whole lot of extra features, many of them optional.
The events directory may also have sub-folders containing their own event files, if one prefers. Each file can hold as many events as one would like. txt file inside the events directory directly below your root mod folder.