1. He stooped down to pick up a block.

2. He bent down to pick up a block.
3. He rounded his body to pick up a block.

(Can we use all the verbs? Do they have the same meaning? Which one is commonly used?)

1 and 2 are correct. They mean the same thing. Both are commonly used.

3 is grammatically correct, but would be an unusual way of saying he bent or stooped. We would not say that. We might say he "bent over" or "stooped over" instead of "down", all meaning the same thing, and commonly used.

1. "He stooped down to pick up a block." - This sentence describes the act of bending forward from the waist in order to reach or grab something from a lower position. It suggests that the person is slightly hunching or bending their upper body.

2. "He bent down to pick up a block." - Similar to stooping down, bending down involves lowering the upper body towards the ground to reach or pick up something. It implies the movement of flexing the body forward at the waist, but without any specific mention of hunching.

3. "He rounded his body to pick up a block." - This sentence implies a more fluid and rounded motion of the body while picking up the block. The phrasing suggests a bending movement that involves more parts of the body, potentially including the back, shoulders, and knees.

All three verbs convey the general idea of bending or flexing the body to reach a lower position and pick up the block. However, there are differences in connotation and usage:

- "Stooped down" and "bent down" are more commonly used and have very similar meanings. "Stooped down" may slightly convey a more pronounced forward hunching or bending of the upper body, while "bent down" is a more neutral phrasing that simply describes the action of bending forward.

- "Rounded his body" is less frequently used to describe this specific action. It could be used in contexts where there is a need to emphasize a more comprehensive or fluid bending movement involving multiple body parts, rather than a simple forward lower.

In everyday conversation, "stooped down" and "bent down" are the more commonly used expressions to describe bending forward to pick up something from a lower position.