beginners most common problem

Tunnel vision, which is a tactical problem that's both a cause and symptom of lack of strategy.

The symptom is tactical.  Say there's some unresolved tension in the center, or a potentially-threatening knight move somewhere on one side.  Beginners have to calculate in those situations, which takes all their mental effort.  You can leave your back rank unguarded and they won't see the mate-in-one.

The solution is to always look at he board with fresh eyes:  is the enemy's king safe?  Is yours?  Once you get in the habit of *looking for* those possibilities, it's not hard to decide that a mate-in-one (or mate-in-three) is more important than an overworked pawn or slippery knight, and you can congratulate yourself on your strategic vision.

moving the a or h pawn 2 spaces and trying to get the rook out early.

not making an attempt to control the center

leaving a hole for me to put an annoying piece and then ruining their entire position in an effort to boot the piece off.  often by over extending with pawns or trading just to put another annoying piece on the same square

allowing an early attack on c7 for black or c2 for white.

not noticing bishops that are far away

only noticing when a piece is protected from behind and not when a piece which has advanced is protecting it

not checking the potential squares of enemy knights

getting forked by pawns when they retreat after over extending

over extending.  retreating, and loosing tempo

By hanging I mean moving a piece to an undefended square where it can be captured or

Opponent makes a direct threat on an undefended piece, player ignores it, so opponent captures it for free.

And as long as the person is... I don't know, older than 14 or 15 I think they can avoid this type of basic blunder through will power (as long as the time control is long enough).

Mariners.  Beginners move to unsafe squares because they do not see it is unsafe; they see their piece is under attack and ignore it for many reasons, and they undefend their pieces and pawns because they are not paying attention; further, age has nothing to do with it. Again it is about training.  They also miss a mate in one in a direct attack against their king. They need to pay attention.  Sometimes it is just stubborness.  I played a very intelligent scientist.  Told him he was hanging a piece.  "But I have a plan."  Once beginners learn what a sacrifice is they want to sacrifice.  What can   I  say?

As a beginner, I wish others would stop playing like this is checkers. Some wanna go piece for piece to only find out I had the upper hand so they resign or auto-resign. Note to self... I have got to get my rating up, so I can play a real game.

Moving their king instead of blocking or capturing in a check.

oie

"What can I say?"

I dunno. Tell them not to sacrifice until they're better I guess 

Yep.

Lack of knowledge and practice. 

Not developing their pieces or developing their pieces at the start of the middlegame.

1. Playing fast time controls.

2. Not being honest about their abilities.

3. Studying material above their skill level.

4. Over dependence on chess engines.

Hanging pieces/ not taking hanging pieces. 

Blundering  mate in one or back rank mates.

Check= mate logic. 

Losing and winning  to or with the Scholar's mate

You see it posted here all the time.  I hear it at tournaments.

"MY CAPs score is....Did i play like a GM?"

"Why was move...only called excellent, when it should have been brilliant"

"I should have won.  I was winning by .4 of a pawn"

1000 players.  Even players at my level should be using engines for 2 things:

1. Blunders

2. Missed tactics

thanks for the advice

1. Moving too quickly

2. not checking for opponent’s threats

3. exchanging for the sake of exchanging and not for a chess-related reason

4. Trying to attack before developing every piece

5. delaying castling