I left out two sentences. Thank you.
1) I take the clothes out (and not: I take out the clothes?)
2) You mustn't speak during the lessons/during lessons/during your school hours/during your school classes.
3) You must be in class when the teacher arrives.
I have to separate dark clothes from light ones (Is that correct?)
I take the clothes out of the dryer.
2 is OK, but parts are wordy. I'd write --
during lessons
during school hours
during classes
3 is OK.
1) To determine the correct word order, you can use the rule for phrasal verbs. In this case, "take out" is a phrasal verb meaning to remove or extract something. Typically, the direct object (in this case, the clothes) comes after the phrasal verb. So, the correct sentence would be, "I take out the clothes."
2) The correct phrase to use in this context would be "during your school classes." It specifically refers to the time when you are attending your classes at school.
3) The sentence "You must be in class when the teacher arrives" is grammatically correct. It means that you should be present in the classroom when the teacher comes in.
Regarding your last question, "I have to separate dark clothes from light ones," your sentence is grammatically correct. It means that you need to divide or sort your clothes, specifically separating dark-colored clothes from light-colored ones.