According to Catholic Teaching, what is sinning against the Holy Spirit? According to Catholic Teaching, what is sinning against the Holy Spirit and why is this unpardonable? [cf. Matt 12:31-32 (RSVCE)]

What are the the sins that are considered sinning against the Holy Spirit?

The desirable answer will be the one that draws from the Church's patrimony e.g. from Church Fathers, Saints &amp; Doctors, and Papal Magisterium.
"Augustine says . . . 


  (Enchiridion lxxxiii) that 'he who dies in a state of obstinacy is
  guilty of the sin against the Holy Ghost,' and (De Verb. Dom., Serm.
  lxxi) that 'impenitence is a sin against the Holy Ghost,' and (De
  Serm. Dom. in Monte xxii), that 'to resist fraternal goodness with the
  brands of envy is to sin against the Holy Ghost,' and in his book De
  unico Baptismo (De Bap. contra Donat. vi, 35) he says that 'a man who
  spurns the truth, is either envious of his brethren to whom the truth
  is revealed, or ungrateful to God, by Whose inspiration the Church is
  taught,' and therefore, seemingly, sins against the Holy Ghost" (ST
  2b:14:2, Sed Contra).


Pope John Paul II writes:


  "Against the background of what has been said so far, certain other
  words of Jesus, shocking and disturbing ones, become easier to
  understand. . . . They are reported for us by the Synoptics in
  connection with a particular sin which is called 'blasphemy against
  the Holy Spirit.' . . . Why is blasphemy against the Holy Spirit
  unforgivable? How should this blasphemy be understood? Saint Thomas
  Aquinas replies that it is a question of a sin that is 'unforgivable
  by its very nature, insofar as it excludes the elements through which
  the forgiveness of sin takes place' (ST 2b:14:3). According to such an
  exegesis, 'blasphemy' does not properly consist in offending against
  the Holy Spirit in words; it consists rather in the refusal to accept
  the salvation which God offers to man through the Holy Spirit, working
  through the power of the Cross. If man rejects the 'convincing
  concerning sin' which comes from the Holy Spirit and which has the
  power to save, he also rejects the 'coming' of the Counsellor . . . If
  Jesus says that blasphemy against the Holy Spirit cannot be forgiven
  either in this life or in the next, it is because this
  'non-forgiveness' is linked, as to its cause, to 'non-repentance', in
  other words to the radical refusal to be converted. . . . Blasphemy
  against the Holy Spirit, then, is the sin committed by the person who
  claims to have a 'right' to persist in evil—in any sin at all . . .
  [T]he Church constantly implores with the greatest fervor that there
  will be no increase in the world of the sin that the Gospel calls
  'blasphemy against the Holy Spirit.' Rather, she prays that it will
  decrease in human souls" (Encyclical Letter Dominum et Vivificantem
  ["The Lord and Giver of Life"] 46-47).


You can read more on this subject here: THE UNFORGIVABLE SIN, James Akin | EWTN.

Summary

The sins that appear here are:


Obstinacy in sin.
Final impenitence.
Rejecting salvation.
Envy at another's spiritual good.





  cf. CCC 1864 "Therefore I tell you, every sin and blasphemy will
  be forgiven men, but the blasphemy against the Spirit will not be
  forgiven." There are no limits to the mercy of God, but anyone who
  deliberately refuses to accept his mercy by repenting, rejects the
  forgiveness of his sins and the salvation offered by the Holy Spirit.
  Such hardness of heart can lead to final impenitence and eternal loss.





  cf. PENNY CATECHISM 326 Which are the six sins against the Holy
  Spirit?
  
  The six sins against the Holy Spirit are:
  
  
  Presumption    
  Despair 
  Resisting the known truth 
  Envy of another's spiritual good 
  Obstinacy in sin
  Final impenitence