Thank you very much.

I need you to check these sentences.

1) My father took a day off work (not a day off from work) to take me to the seaside.
2) They took 20% off (not "from") the list price.
3) He let the students off with an warning. He let off the students with a warning.

1. Either phrasing is correct.

2. yes "off" is correct

3. 1st one is OK; 2nd is not.