相当【xiāngdāng】

相当 goes before an adjective and says the quality is there to a fair, even surprising degree: "quite", "rather". It's stronger than 比较.

It's more formal than 挺, and suits writing and careful speech. It works for good and bad qualities alike.

Before a verb

Verbs of feeling, and verb phrases with a degree in them, take 相当 too.

Before a noun

相当 + 的 + noun means "a fair amount of": 相当的时间, "quite some time".

相当 already carries the degree, so it doesn't stack with 很 or 非常.

• × 这个问题很相当难 → 这个问题相当难

相当于, "is equivalent to", is a different word with its own point.