"Unless" is a conjunction that is commonly used in English grammar to express a condition that must be met in order for something to happen. It is often used in negative conditional statements.

Here are some examples of how to use "unless" in a sentence:

1. I won't go to the party unless my friends are going too.
2. You won't pass the test unless you study hard.
3. Unless you apologize, I won't forgive you.

In all of these sentences, "unless" is used to introduce a condition that must be met in order for the main clause to be true.

double negative

The sentence "unless he does not return" is not considered grammatically correct in English because it contains a double negative. The word "not" is a negative word, and when combined with "does not," it creates a double negative that can be confusing.

To express the same idea without using a double negative, you could rephrase the sentence as "unless he returns," which conveys the same negative condition without using two negative words.

Here are some more examples of how to correctly use "unless" in a sentence:

• Unless you eat your vegetables, you won't get dessert.
• I won't be able to come to the party unless I finish my work.
• Unless it rains, we'll have a picnic in the park.

In all of these examples, "unless" is used to introduce a negative condition that must be met for the main clause to be true, without using a double negative.

in English grammar, it is not common to use a negative word after "unless". This is because "unless" already introduces a negative condition, and adding another negative word can create confusion and make the sentence more difficult to understand.

In your example, "unless he does not return" is not correct because it contains a double negative. A better way to phrase this would be "unless he returns", which expresses the negative condition that must be met for the main clause to be true.

Here are some more examples to illustrate this point:

Incorrect: Unless I don't finish my work, I won't go to the party. Correct: Unless I finish my work, I won't go to the party.

Incorrect: Unless she doesn't wake up early, she will miss the train. Correct: Unless she wakes up early, she will miss the train.

In both of these examples, the incorrect sentence contains a double negative, while the correct sentence only has one negative word (introduced by "unless").

it is true that in many cases, using the present tense or present perfect tense after "unless" can make the sentence clearer and easier to understand, especially when expressing a negative condition that is currently relevant.

Using the future tense after "unless" is not incorrect in English grammar. However, when using "unless" to introduce a negative condition, it is more common to use the present tense or the present perfect tense, rather than the future tense.

Here are some examples to illustrate this point:

• Unless I finish my work, I won't be able to go to the party. (present tense)
• Unless she has studied, she won't pass the test. (present perfect tense)
• Unless they arrive soon, we will have to start the meeting without them. (future tense)

In the first two examples, the negative condition is expressed in the present tense or present perfect tense, while in the third example, the negative condition is expressed in the future tense. All of these sentences are grammatically correct, and the choice of tense depends on the specific context and meaning you want to convey.

However, it is true that in many cases, using the present tense or present perfect tense after "unless" can make the sentence clearer and easier to understand, especially when expressing a negative condition that is currently relevant.