Independent variable — the one thing you deliberately change. Exactly one, unless you are running a factorial design on purpose.
Dependent variable — what you measure. It may respond to your change; that is what you are testing. State its units and the instrument.
Controlled variables — everything you deliberately hold constant. List them explicitly with values: "light 12 h/day at 200 µmol m⁻² s⁻¹", not "same light".
Confounding variables — things that vary along with your independent variable without your intending it. These are what randomisation and blinding exist to catch.
Worked example. Does salt concentration affect radish germination?
- Independent: NaCl concentration (0, 25, 50, 100 mM) — four levels, one variable.
- Dependent: proportion of seeds germinated at 7 days.
- Controlled: seed lot, temperature 21 °C, 10 mL solution per dish, same paper substrate, same dark cabinet.
- Potential confound: dishes at the front of the cabinet are warmer. Fix by randomising dish position and rotating daily.
If you cannot name the confound you are most worried about, you have not looked hard enough yet.