To allow, not to prevent (+ infinitive, usually without to).
- After he knocked for hours, I decided to let him come in.
To allow the release of (a fluid).
- The physicians let about a pint of his blood, but to no avail.
To allow possession of (a property etc.) in exchange for rent.
- I decided to let the farmhouse to a couple while I was working abroad.
To cause (+ bare infinitive).
- Thenne the kyng lete serche how moche people of his party ther was slayne.
- Time's sea hath been five years at its slow ebb, / Long hours have to and fro let creep the sand.
- Can you let me know what time you'll be arriving?.
To hinder, prevent.