I just started playing NL50 HU and first two sessions went well. This hand really confused me though. It was the first hand against this guy but I never saw the river C/R coming.

Full Tilt Poker $0.25/$0.50 No Limit Hold'em - 2 players
The Official 2+2 Hand Converter Powered By DeucesCracked.com

Hero (BTN/SB): $142.40
BB: $50.00

Pre Flop: ($0.75) Hero is BTN/SB with J K
Hero raises to $1.50, BB calls $1

Flop: ($3.00) K 4 7 (2 players)
BB checks, Hero bets $2, BB calls $2

Turn: ($7.00) 3 (2 players)
BB checks, Hero bets $4.50, BB calls $4.50

River: ($16.00) 8 (2 players)
BB checks, Hero bets $12, BB raises to $42 all in

I understand I underbet the turn, don't know why I did that, but I just can't figure out where this raise came from. Did he turn the straight? Slowplay a set? Get lucky with K8? Bluff? This guy ended up taking a chunk of my stack and had a pattern of big river bets that I could never call.

easy fold

This is a fold without more info that hes an aggro nut who can do this as a bluff. 78, 56, K8, 88 are all possible.

PTR the villan if hes spew/new player its a call if hes not its a fold

the value range of even the spewiest players ever crushes us here. and it's pretty unlikely that he double floats us OOP with air to c/r the river. so it's just pretty hard for him to get to the river like that with air on such a dry flop to start with. and most players are not capable of turning pairs into bluffs at 50nl so this is a really trivial fold imo.

well played, now fold. maybe bet less on river.

River bet is fine imo. Fold though.

river size is good

well played, now just fold on river. could be a call vs someone tricky or spazz. w/o too this reads, just b/f river.

I did PTR the guy before the hand and he was small loser over a small sample size but even if he was a huge loser over a huge sample size, I don't think I could strongly consider that and make a hero call here. There's no draws that he's missing unless he floated me with a backdoor heart draw.