I observed that Following main Questions which are frequently asked usually tagged with apt or software-installation with command-line or dependency:

• Unable to locate package:

   E: Unable to locate package [pkg]
  

• Package has no installation candidate:

   E: Package '[pkg]' has no installation candidate
  

• Packages have unmet dependencies:

   The following packages have unmet dependencies:
   package1 : Depends: package2 (>= x.y) but [pkg] is to be installed
   E: Unable to correct problems, you have held broken packages.
  

Where [pkg] means package-name.

From which we have already Questions as follows:

• For Unmet dependencies: How do I resolve unmet dependencies after adding a PPA?
• For Unable to locate package I've prepared (which needs to improve): Unable to locate package

For new users of Ubuntu and command-line, further method of installing software properly,

It needs to prepare FAQ type wiki/solution on above (3) Main/Considerable Q/A.