Brake pads have wear indicators — small metal tabs that contact the rotor when pad material is near its end. The contact produces a high-pitched squealing sound under braking.
Squealing during braking is the early warning. You have time (typically 1,000–2,000 miles) to schedule replacement.
Grinding during braking is the late warning. Pad material is gone, and metal-to-metal contact is damaging the rotor. Replace immediately — continued driving may require rotor replacement too.
Soft, spongy brake pedal: brake fluid issue, not pads. Could be a leak or air in the lines. Get inspected.
Pulling to one side during braking: usually a caliper issue or uneven pad wear. Inspect both sides.