凡是…都【fánshì…dōu】

凡是 goes at the very start and names a whole category: "every … that", "anyone who". The second half says what holds for all of it, and 都 marks that sweep.

What follows 凡是

The category is usually a noun with a description in front of it – a verb phrase + 的 + noun – so it reads "every X who/that…".

都, 就 or 一律

都 is the default. 一律 sounds like a rule and suits notices. 就 links a condition to its result: "whenever it's X, then Y".

Dropping 都

English "everyone who" needs nothing in the second half. 凡是 does: without 都 (or 就, 一律) the sentence feels unfinished.

• × 凡是学生可以参加 → 凡是学生都可以参加

凡是 also stays at the front, before the category, never after a subject.

• × 学生凡是都可以参加 → 凡是学生都可以参加