Greedy by earliest start time: always pick the activity that starts earliest.
Example: (0,10), (1,2), (3,4). Greedy picks (0,10) first. Now (1,2) and (3,4) both conflict with (0,10). Greedy gets only activity. But the optimal is (1,2), (3,4) for activities. Starting early does not help if the activity runs too long. It blocks better options.
The problem: you commit to a long activity before seeing what it blocks. This greedy strategy fails because start time does not capture opportunity cost.