"Mandatory" means that the thing must be done due to some reason or rule.
These courses are mandatory.
That is, you must take the courses or else you fail.
Wearing helmets was made mandatory a few years ago.
It is a rule that you must wear helmets.
"Indispensable" is that the thing is so critical, useful, or important that you cannot conceivably throw the thing away.
The volunteers' help was indispensible.
The mission would have failed if the volunteers didn't help.
He made himself indispensible to the parish priest.
He was deemed a must-have person because of some good quality.
These great tools are so versatile that they are indispensable.
The tools are so good that you shouldn't be without them.
To sum up, these two words are not interchangeable - they have their own distinct meanings.
The nuance can be demonstrated with a couple of examples:
I usually have a cup of coffee in the morning.
I've often heard that pop song on the radio.
often expresses frequency. usually expresses the customary. What is customary occurs with some frequency, but what is frequent is not necessarily customary.
Best Answer
They are basically the same thing in regular usage.
The milk in my coffee is frothy. The coffee has froth on the top.
The milk in my coffee is foamy. The coffee has foam on the top.
The ocean was foaming from the storm. or frothy or foamy.
Those are positive in meaning.
The difference resides mostly in technical or medical usage:
Spray the foam into the space between the two strips of wood and then press them together.
Foam there is the form of a particular substance: a foam as opposed to a liquid. So, glue could be in the form of a foam, rather than liquid.
The man had been poisoned and froth had come out of his mouth.
Froth is used to refer to the spittle that comes out of a person's mouth, usually, if they are sick or have been poisoned, for example.
Froth and foam are also used to describe anger: He was so angry he was frothing or foaming at the mouth. [spittle was coming out of his mouth].