°F = °C × 9/5 + 32; °C = (°F − 32) × 5/9. Body temperature 37°C = 98.6°F.
Kelvin is the absolute temperature scale used in physics and chemistry. 0 K = −273.15°C; there are no negative Kelvin values.
Yes. The same formula applies. Notably, −40°C equals −40°F — the one temperature where both scales agree.