I hear 强调过去发生的事对现在的影响，即此事还在眼前，等同于I’ve heard. 一般过去时的I heard则侧重过去就听说的什么事。麦克米伦词典还特意将I hear和I’ve heard 划为同一词条，即说明I hear等同于现在完成时的I’ve heard. 请看词典释义：

I hear/I’ve heard

used for introducing a subject that you want to discuss with someone that someone else has told you about

例句：I hear you’re leaving us.( 我听说你要走了）

https://www.macmillandictionary.com/dictionary/british/i-hear-i-ve-heard

除此之外，I hear 在某些语境还有其它含义:

例如：

1. I hear you won a lottery. 我听说你中大奖了。（言外之意说话人可能是道听途说，不一定准确）。

2. I hear what you say, but unfortunately the deadline cannot be extended. 我明白你的意思，（言外之意你甭跟我解释），截至日期不能延长。