Why Are Metals Hard? Give The Uses Of Some These Selected Metals Such As Aluminum, And Brass.

Metal have lots of free electrons which create a huge electron pressure (on the order of 100,000 atm), so in order to crush them, you'd have to expose them to pressures higher than their internal electron gas pressure; this is why they're hard.  In fact, the only reason you can bend them is cause they have missing atoms (dislocations) in their crystal stucture.  Learn more about them here: resources.schoolscience.co.uk/corus/16plus/steelch1pg5.html

Metals consist of a sea of electrons that can move from one place to another in the crystal. Each metal cation attracts all the de-localized electrons. This result in release of energy and the nuclei are bound together by a strong bond. This is the reason why metals are so hard.
Coming to the uses of the metals mentioned in the question.
Aluminium is a very useful metal and is has a lot of uses in our daily lives. Some of them are as follows
It's a very light metal and resistant to corrosion therefore it is used packaging food in aluminium tins.
Its property of being a good conductor of heat and resistant to corrosion makes it useful metal to be in the making of cooking utensils such as saucepans, spoons etc.
It's a good conductor of electricity and is cheaper than copper hence it is used to make electric transmission wires.
Its alloys are light an have a high tensile strength, therefore they are used in the construction of aero plane bodies and some motor car parts.

Uses of brass
It is an alloy of copper and zinc. It is resistant to corrosion and has a pleasing colour therefore it is used to make decorative articles, water taps and copper pipes