once 后面加现在完成时什么意思

once后既可以使用一般现在时，也可以使用现在完成时。
例如：
Once you start,you will never give up.一旦你开始了,你就不要放弃.
Once you begin to read it,you will like it.一旦你开始读,你就会喜欢它.
Physics is easy to learn once you understand the rules.一旦你理解了规则,物理就不难学了.
Once you have learned how to swim,you will never forget it.
Once you have been working from home for a while,you might feel a bit lonely.