中文：

股票挂单是投资者在股票市场进行交易前必须执行的操作之一。挂单意味着投资者向交易所提交一个买卖指令，以在未来的某个时间点以特定的价格买入或卖出股票。

那么，股票什么时候可以挂单呢？实际上，投资者可以在任何时候进行挂单，只要他们有意愿并且具备相应的交易权限。然而，需要注意的是，交易所对于不同交易品种的挂单时间有一定的规定。一般来说，交易所在开盘期间接受挂单，而在收盘期间则不再接受新的挂单。此外，投资者还应当注意各个券商的交易系统和平台，因为不同的交易系统可能有不同的挂单时间和规则。

挂单可以帮助投资者更好地掌握交易的主动权，并且可以提前设定买入或卖出的价格。然而，投资者也应当注意风险和机会的平衡，因为市场波动可能会影响挂单的执行。

英文：

Order-placing is one of the necessary operations for investors to carry out trading in the stock market. Order-placing means that investors submit a buying or selling order to the exchange at a certain price point in the future.

So, when can investors place orders for stocks? In fact, investors can place orders at any time as long as they have the intention and the corresponding trading privileges. However, it should be noted that the exchange has certain regulations for the order-placing time of different trading varieties. Generally speaking, the exchange accepts orders during the opening hours, but no longer accepts new orders during the closing hours. In addition, investors should also pay attention to the trading systems and platforms of different brokerage firms, because different trading systems may have different placing time rules.

Placing orders can help investors better grasp the trading initiative and set the buying or selling price in advance. However, investors should also pay attention to the balance between risks and opportunities, because market fluctuations may affect the execution of orders.