The correct Hârnic titles are given in HârnPlayer, in Hârnic.
However, as an English speaker, I prefer to use English. I would go further than T.Devon Sharkey. From the bottom up:
The unfree have no titles (as befits their status)
Freemen I refer to as:
Freeman Braeburn, for a farmer or other unguilded freeman,
Yeoman Kilgour, for a yeoman
Guildsmen are referred to by Guild rank a master miller would be referred to as Master Miler, or Master Thaly.
Within towns there are of course also Aldermen and in many cases Mayors.
The nobility like titles, the more the better:
Landless knights, such as Sir Lluwyn of Haber, are referred to as Sir Lluwyn (never as Sir Haber) this is a particular bugbear of mine
Landed Knights, such as Sir Aspin of Martaryne should properly be referred to as Lord Rudwyn (Rudwyn being the manor he holds). Social inferiors would call him milord, equals can choose to use Lord Rudwyn or Sir Aspin. A superior would also use these, but could simply call him Aspin. This is a bit of an insult, but they're entitled (in the literal sense of that word) to do it.
A Baron, such as the Baron of Getha, would properly be referred to as his Excellency Sir Chimin Indama, Baron of Getha. Lessers refer to him as your Excellency or Baron Getha. Equals can choose Baron Getha or Sir Chimin, as with Landed knights above.
An Earl, such as the Earl of Balim, would properly be reffered to as His Grace Sir Troda Dariune, Earl of Balim, Baron of Kyg, Baron of Jedes (In my p-Hârn I assume that where keeps are held by constables for Earls the title Baron is retained by the Earl). This rule also applies to Barons with respect to manors they hold directly through bailiffs. This makes formal introductions extremely long winded as nobles make sure that everyone knows the full extent of their land holdings.
I suspect that heirs, once knighted, would be given one of their parents titles. This would allow The Earl of Balim to confer one of his lesser titles (say, Baron of Kyg) to his son and heir Scina (sorry your excellency, I meant to say his excellency Sir Scina Dariune, Baron of Kyg (tugs forelock)). However, this may be rather problematic on Hârn where clan councils can determine that the eldest son does not inherit.
Certainly in the UK various titles belong to children as a right. The Kings/Queens eldest son is always the Prince of Wales. (IIRC he has been since the black prince, I'm sure our experts will correct me if I'm wrong).