Learn German: Jeder kennt das Geheimnis. - Everyone knows the secret.

Jeder translates directly to "everyone" or "everybody." Just like in English, it is grammatically singular, so it takes the singular verb kennt. Alle means "all" and is plural. If you used it, you would say Alle kennen das Geheimnis (All know the secret).

Why does "jeder" end in "-er" here?

When used on its own as an indefinite pronoun to mean people in general, it takes the masculine singular nominative ending -er. This is the standard default form for "everyone."

Why use "kennt" instead of "weiß" to say someone knows a secret?

German has two verbs for "to know." Wissen is used for knowing facts, often followed by a clause (like "I know that..."). Kennen means to be familiar with a person, place, or thing. Since a secret is a noun (das Geheimnis), you use kennen to say you are familiar with it.

Is there a trick to remembering the gender of "das Geheimnis"?

Yes! Many nouns that end in the suffix -nis are neuter (das), such as das Erlebnis (the experience) or das Ergebnis (the result). There are a few feminine exceptions, but neuter is your best guess for -nis words.