The Hercules Knot more commonly known as Reef Knot is one of the most important stronghold knots. It is easy to remember with a rhyme " Right over Left, Leftover Right makes the knot more Tidy and Tight.
It is used to secure the rope or line around the object. It is not at all recommended to use it for tying two ropes together because of the potential instability of the knot which has caused many accidents over time.
Clove Hitch or Double Hitch is one of the most important knots, it is basically two half hitches around an object.
It is used when the length of the running end needs to be adjusted, mostly clove hitch is used in modern climbing. Remember clove is not much reliable on the rectangular or square object as its grip best fits with round objects.
Sheet Bend is quick and easy to tie a knot, it is so essential that no matter where you look it will be listed in most essential & must learn knot.
it is used to join lines of the same or different diameter or rigidity. The sheet bend is related to Bowline it has a tendency to work loose when not under load. In order to increase security it is sometimes recommended to add another turn in the smaller end.
The bowline is an ancient and simple knot used to form a fixed "eye" at the end of a rope. It has the virtues of being both easy to tie and untie; most notably, it is easy to untie after being subjected to a load. The bowline is sometimes referred to as King of the knots because of its importance.
The taut-line hitch is an adjustable loop knot for use on lines under tension. It is useful when the length of a line will need to be periodically adjusted in order to maintain tension. It is made by tying a rolling hitch around the standing part after passing around an anchor object. Tension is maintained by sliding the hitch to adjust size of the loop, thus changing the effective length of the standing part without retying the knot.
It is typically used for securing tent lines in outdoor activities involving camping, by arborists when climbing trees, for tying down aircraft, for creating adjustable moorings in tidal areas, and to secure loads on vehicles. A versatile knot, the taut-line hitch was even used by astronauts during STS-82, the second Space Shuttle mission to repair the Hubble Space Telescope.