She is sitting in the back of the car.

She is sitting in back of the car.

She is sitting in the front of the car.
She is sitting in front of the car.
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What is the difference between them?
Do we have to use 'the' or not?
Is she inside the car or ouside the car?
Can we use 'at the back of' instead of 'in back of' in this case?

In the back of the car - means that she is INSIDE the car.

In back of the car - means that she is sitting outside the car and behind it.

The same difference would apply to " in the front" and " in front of"

Actually we usually say, She is sitting in the back seat, or she is sitting in the front seat. (if the person is inside the car)

Thank you.

What about the following?

She is sitting at the back of the car.
She is sitting at back of the car.

She is sitting at the back of the car.... would mean that she is outside the car and is sitting in a chair or on the ground behind the car....

At back of the car is not used.

The difference between the sentences is the use of the article "the" before the preposition.

In the first set of sentences, "the" is used, which indicates a specific location within the car. "She is sitting in the back of the car" means that she is seated in the designated area at the rear of the car.

In the second set of sentences, "the" is omitted. This implies a more general sense of location. "She is sitting in back of the car" means she is seated behind or outside the car, without specifying a particular area.

To determine whether she is inside or outside the car, consider the context. If she is "sitting in the back of the car," it suggests that she is seated inside the car, specifically toward the rear. Conversely, if she is "sitting in back of the car," it implies that she is positioned behind or outside the car.

Regarding the use of "at the back of" instead of "in back of," they have similar meanings, but there are some subtle differences. "At the back of the car" suggests a specific location within the car, while "in back of the car" generally indicates a position behind or outside the car. However, the difference in meaning is quite minimal, and in many situations, these phrases can be used interchangeably.