As already said it's easy to spot a tactical danger.The really difficult is to realise how dangerous it is.
It is black's move and he does his tactical evaluation.What are the dangers for black's position?
At the heat of the battle a check can seriously alter the outcome of the game.I n many cases the player preparing the attack spends a move to hide his king so that to avoid unpleasant surprises.When your king or opponet's king is exposed to checks try to find the possible crossed diagonals double attacks(CDDA).It has been proved that all players of all levels tend to miss or evaluate wrongly the CDDAs, even more than the knight fork double attacks.Beginners have difficulty seeing fork attacks but as they become more experienced ,they become more proficient in dealing with them( the word "knight" for every chessplayer is the synonym of "danger", we gradually become more alert to them).CDDAs though remain a serious problem not because we can't see them but because most of the times we tend to underestimate them.That is more a problem in OTB games where the limited time makes players to set priorities and examine deeply only what they consider really dangerous.
Black's king is on g8.Black has to carefully find the possible checks.The danger is obviously from the g8-a2 diagonal and the only piece that can give a diagonal check is the queen:
1)A check from a2 is harmless.
2)A check from b3 is also harmless now but if black plays ...c6(a usefull move) , then Qb3+ will gain b6-pawn(important point to remember)
3)A check from c4 would gain c7 pawn(also important point to remember as Qe6 can't move to any other square except e7 ,d7 and f7)
4)A check from d5 would gain Ra8(important point to remember)
Black has to be alert for cases 2 , 3 and 4.From these cases , the 4th seems to be the most impossible as d5 is defended by Nf6 and Qe6 and even if these 2 leave , Ra8 is protected by Rf8 , so even if white is able to play at some point Qd5+ , he won't be able to gain anything as Ra8 is still safe.
Is it possible 3 pieces(Qe6,Nf6 and Rf8) to be neutralised with one move?
Unfortunately , for Black, it is.
Black underestimated the danger and lost.You may think that it was his blunder at 21st move (21...Nh8??)that made him lose but that is not true.After 21.Raf1 black is forced to move his f6-knight because of the Rxf6 tactical threat and that puts his position under pressure.The equal at move 20(after 20.Rf3 position) becomes clearly better for white after 21.Raf1.
The solution was very simple and very obvious from the tactical evaluation.Can you see it?
Which should be black's move instead of the wrong 20...Nf7?
Avoiding tactical blunders is a major issue for every player , not only beginners.No matter how easy or how difficult a combination is, it's features are always very simple and easy to spot.
It will be always a file a diagonal or a knight fork that will create a , usually decisive, double attack .Knight forks are a special case that need to be studied separately.
We will try here to learn to recognise the dangers files and diagonals can create.
To avoid tactical blunders it is necessary to learn to do Tactical Evaluation of the position every move.After every of your opponent's moves , learn to examine the position carefully for tactical threats and learn to do safety check on your position when choosing a move.