1. Stand in line, please.

2. Stand in a line, please.
3. Stand in two lines, please.

4. Stand in row.
5. Stand in a row.
6. Stand in two rows.
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Can we use all the expressions?
What is the difference between them?

1 answer

All are fine.

I think of a line as people standing one-by-one, in back of each other.

I think of a row as people standing next to each other, shoulder to shoulder.