Questions & Answers about Поставь стул к столу, пожалуйста.
Поставь is the imperative (command/request form) for ты (informal you) from the verb поставить.
- поставь = (you, informal) put / place (it)
- поставьте = (you, formal or plural) put / place (it)
There is no standard imperative постави for this verb in modern Russian.
Requests like this often use the perfective imperative to ask for a single completed action: move it and be done.
You can also hear imperfective in other contexts, but it changes the feel:
- Поставь стул к столу (perfective) = put the chair by the table (one concrete result)
- Ставь стул к столу (imperfective) can sound like: go ahead and (habitually/ongoingly) put it there, or instructions while the action is in progress. The perfective is the natural default for a one-time request.