Where to use conservation of energy? I have seen many problems in physics where the solution starts wih "using conservation of energy, we get" without telling anything about why do we start with like this,
My concern is how do we know that we should use Conservation of Energy in a question ?
Is it like when we have to calculate velocity, distance, force we should use this law ? Or when there is motion of objects?
is it when time is not asked?
What could be the tag words for the same as otherwise solving physics problems becomes a guess work and goes into a rote study like history and geography that you have to solve the same problem previously to solve it which is not possible.
 A: The Conservation of Energy holds almost all the times but the thing is we can't use it to calculate "what portion of the energy is distributed in various forms ?" e.g. If you push a  block resting on a table , the block moves and then stops after some distance.
Now if you touch the block, you can feel that it's hot. Also while it was moving you might heard some rubbing sound.
So basically using the Conservation of Energy you can tell that the whole initial kinetic energy is lost in other forms (heat, sound, electrical sparks) but you can't generally tell how much is lost as sound and how much as heat.
But with forces which are non - dissipative in nature (conservative forces) you don't have to worry about such losses and you have to just account for the Potential energy and kinetic energy which are easy to calculate.
So the thing is to recognise the force in action and then decide whether you should use it or not.
A: You can use conservation of energy in any scenario where you can calculate the value of energy in all forms both at the beginning and the end of the time period in question.
You cannot use conservation of energy in problems where an unknown amount of energy is dissipated into the environment as heat, sound, or deformation of objects. This will be the case in any problem that involves friction or an inelastic collision. Note that energy is still conserved in these scenarios, but there is a term in the “before=after” energy equation that has an unknown value.
A: 
Where to use conservation of energy?

You use conservation of energy whenever its applicable and its the most convenient method of calculating whatever you want to calculate. Conservation of energy is just another tool in your tool chest just like the equations of motion. Maxwell's' equation, Kirchhoff's' equation etc.
For most problems there are multiple ways to solve them. You can go with whatever feels easiest to you. This will depend on the problem, your skill sets and your preferences.
"Conservation of energy" is in many problems a good one to start with since the math tends to be on the easier side.  If in doubt, try a few methods and choose the one that feels easiest to you.
A: In the universe, “Conservation of total energy” holds.
If “the system is appropriately chosen (so that there is no addition or removal of matter and energy) and that you are able to account for all but one of parameters that appear in the resulting equation, thus leaving one parameter to solve for”, 
then “Conservation of total energy” is useful…and we can use it to help solve our problem.
(At this point, one should consider various example situations.)
A: For any closed system where no energy is being added or taken out, the conversation of energy holds. In most high school/low undergrad physics problems, the systems studied are mechanical systems like blocks, particles, etc. under the influence of forces including mechanical forces, gravity, electromagnetism, etc.
Energy is conserved under the influence of gravitation and electromagnetism because they are conservative forces, which is something you may want to look into.
