White’s pawn on d4 has no pawn on the c-file and none on the e-file. No pawn will ever defend it. If it comes under attack three times, it has to be defended three times, and each defender is a piece that would rather be doing something else.
7 
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g The square in front matters more
Look at d5 rather than d4. Because White has no c-pawn and no e-pawn, that square can never be attacked by a white pawn, so a black knight there is on an outpost for the rest of the game. The pawn is a target you can defend. The hole in front of it is a gift you cannot take back.
That is the pattern with every isolated pawn: the pawn is weak, and the square immediately ahead of it is weaker. Blockade the square first with a knight, and only then start attacking the pawn with everything else. Once the pawn is frozen it cannot run away by advancing, which is the one way an isolated pawn can solve its own problem.
Why anybody would want one
Strong players walk into isolated pawns on purpose. The Tarrasch Defence hands Black an isolated d5 pawn straight out of the opening, and the Queen’s Gambit Accepted often gives White one on d4.
What you get is space, because the pawn stands on the fourth or fifth rank and takes squares away, plus two half-open files, the c-file and the e-file, with a rook on each. The pawn also supports a knight on the square diagonally in front of it, e5 for White. So the deal is: attacking chances now, a bad endgame later. Which is why the side with the isolani plays for the attack and the other side trades pieces at every opportunity.
I tell students to take the pawn on if they like open positions and avoid it if they enjoy grinding, because the structure chooses your plan for you before you have thought of one.
Telling isolated from backward
They are different weaknesses. An isolated pawn has no neighbours at all; a backward pawn still has one, but it has fallen behind and cannot catch up. Both end up on a half-open file getting stared at by rooks, and both are worth avoiding when a capture gives you the choice.
The attacking half of the isolani story is worth practising directly: most of the games are decided by a piece landing on e5 and a rook swinging over, which is exactly what the kingside attack puzzles drill.