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?

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.

1. "Stand in line, please." - This expression is commonly used in American English and is often used when there is a single line for people to stand in. For example, if you are waiting to order food at a counter, you might hear someone say this.

2. "Stand in a line, please." - This expression is also commonly used in American English and means the same thing as "Stand in line, please." The only difference is the addition of the indefinite article "a" before "line." Both expressions are typically used interchangeably.

3. "Stand in two lines, please." - This expression suggests that there are two separate lines for people to stand in. It is used when there is a need for a division of people or when there are too many people to fit into a single line. For example, at a bus stop, if there are two buses departing from different places, you might be instructed to stand in two lines based on your destination.

4. "Stand in row." - This expression is not commonly used in English, especially in American English. It may sound unusual or awkward to native English speakers.

5. "Stand in a row." - This expression is similar to "Stand in line," but instead of a single file line, it suggests a formation of people standing side by side in a straight line. It is often used in situations where people need to be evenly spaced, such as during a military drill or a group photo.

6. "Stand in two rows." - Similar to "Stand in two lines," this expression suggests having two straight lines of people standing side by side. It is also used when there is a need to divide a large group into two smaller groups. For example, if you are organizing a classroom and there are too many students to fit in a single row, you might ask them to stand in two rows.

It's important to note that the usage of these expressions may vary depending on the context and regional dialect.