The past tense of keep your nose to the grindstone is kept your nose to the grindstone.

The third-person singular simple present indicative form of keep your nose to the grindstone is keeps your nose to the grindstone.

The present participle of keep your nose to the grindstone is keeping your nose to the grindstone.

The past participle of keep your nose to the grindstone is kept your nose to the grindstone.