1. You can write everything down.

2. You can write down everything.
(Are both correct?)

2-2. You can write it down. (O)
2-3. You can write down it. (X)

(When the object is a pronoun, we cannot write 'write down it', right? Then, what about #2? In #2, everything is a pronoun. Is #2 correct?)

3. Turn your cellular phone on.
Turn your cellular phone off.
Turn on your cell phone.
Turn off your cell phone.

(Are all correct?)

The first question: both are correct

The second question: huh?
The third question: all are correct

1 and 2 and 2-2 are correct; 2-3 is not correct.

As Nancy said, all in 3 are correct.

1. Both sentences are correct. They have the same meaning and can be used interchangeably. "Write everything down" and "write down everything" both mean to record or put in writing all the necessary information or ideas.

2-2. "You can write it down" is correct. In this sentence, "it" is a pronoun referring to a specific object or idea. It is grammatically acceptable to say "write it down" because the pronoun "it" comes before the verb "write."

2-3. "You can write down it" is not correct. When the object is a pronoun, it should come before the preposition "down." Therefore, saying "write down it" would be incorrect. Instead, you should use the pronoun before the verb, as in "write it down."

3. All four sentences are correct and convey the same meaning. They are different ways of instructing someone to activate or deactivate their cellular phone. You can choose any of these variations depending on your preference or the context of the communication.