Legal Writing

If I’d know this years ago.

This is a non-standard use of the verb, possibly an example of dialect. In standard usage known is the past participle called for in this clause: If I’d known this years ago?

Know belongs to a small group of verbs that have retained their irregular forms:

know-knew-(have) known
blow-blew-(have) blown
grow-grew-(have) grown
throw threw (have) thrown

The third form in each example is the past participle form used with the helping verbs have and has. The past participle can also be used as an adjective. Here are some examples:

Verb use
If I’d known this years ago, I would have changed my behavior.
The wind has blown without cease for three days.
I have grown these tomatoes from seed.
Billy has thrown a perfect curve ball.