..は, ...が => Compare は and が N5
Compare
が  => but N5
Contrast
が => But... N4
Contrast
が => Well, then N4
How to say the beginning
が => however; but ~ N5
が => Auxiliary words N5
Auxiliary words
だが => But N2
~ようが...まいが => whether or not; regardless N1
... だろうが、... だろうが => Whether it's ... or ... N3
Demonstrate by example
~のが      => Nominalize verbs N5
Nominalization
〜がさいご(が最後) => if you do... negative result N1
Decisive (resolute decision)
~んがため(に)/~んがための      => in order to ~ N1