The synopsis is part of the book

A synopsis is not a school exercise. It is the book told to a busy stranger who has forty other stories in the pile. They are not hoping to be surprised by your refusal to spoil the ending. They are hoping to see that the story has an ending, and that the person who wrote it can tell it without getting lost.

Write it in the present tense, in order, naming the turns. Who wants what, what blocks them, what it costs, how it resolves. One page is a courtesy. Two pages is often the limit of goodwill. Theme words and character sketches that do not change the plot can go. The stranger can feel the theme if the events are the right events.

The voice of the book may tint the synopsis, lightly. A comedy should not be summarised like a coroner's report. But this is not the place to show off a style that obscures who did what. Clarity is the style. If you cannot tell the story plainly, you may not yet know what the story is, which is useful news, got cheaply.

Do not hide the ending. A synopsis that stops at 'and then everything changes' reads as a writer who hopes the agent will request the manuscript out of curiosity. Some will. Most will assume you do not know how it ends, because many writers do not.

Write the synopsis when the draft can answer the questions, then keep it with the submission letter as part of the same craft. The book is the long version. The synopsis is the proof you can hold the long version in your head.