突然【tūrán】

突然 says something happened without warning. It goes before the verb, or at the very start of the sentence with a comma after it.

The change has usually happened, so the sentence tends to end in 了 or carry a complement like 起来.

As an adjective

突然 is also an adjective, "sudden". It takes 很 or 太 like any other adjective, and goes before a noun with 的.

忽然 also means "suddenly", but only before a verb; it can't be an adjective.

• × 这件事很忽然 → 这件事很突然

Not at the end

"Suddenly" can close an English sentence. 突然 can't.

• × 车停了突然 → 车突然停了