Todo and cada: all, every, each

Todo: the whole, every, everything

Todo agrees fully and, before a noun, usually takes the article (or another determiner, as in todos mis amigos, all my friends). Singular means the whole; plural means every:

Two fixed uses to learn whole:

• Todo el mundo lo sabe. - Everybody knows it. Singular verb: todo el mundo is one group.
• Lo quiero todo. - I want it all. On its own, todo means everything.

Cada: each, and every two hours

Cada never changes and takes a singular noun with no article:

The last row is the one job only cada can do: an interval with a number, where the noun is plural.

• Tomo la pastilla cada ocho horas. - I take the pill every eight hours.

Cada día and todos los días both mean every day. Cada takes the days one at a time, so it suits a change from day to day:

• Cada día hace más frío. - It gets colder every day.
• Voy al gimnasio todos los días. - I go to the gym every day.