首先我们来看下happen和happen to的大致意思：
happen：词性为动词，to take place or occur; come to pass.
happen to：词性为动词短语，to have the fortune or chance (to do something); come about or suddenly appear in a particular way or place.
通过下面的表格我们了解下happen和happen to的含义、发音和用法
接下来让我们看下happen和happen to的用法区别：
1.词义：happen用于描述事件或情况的发生，表示偶然性；happen to用于描述某人或某物在某个时间或地点发生了意想不到的事情。
例子：
- It happened to rain yesterday.
(昨天下雨了。)
- I happened to meet him on the street.
(我碰巧在街上遇到了他。)
2.语法结构：happen是及物动词，后面可以直接接名词或代词作宾语；happen to是不及物动词短语，后面必须接名词或代词作宾语，并且to为介词。
例子：
- His wedding happened to be canceled suddenly.
(他的婚礼突然取消了。)
- I happened to see him yesterday.
(我昨天碰巧看到了他。)
3.用法：happen多用于正式场合，描述一般的事件发生；happen to多用于口语和非正式场合，描述偶然发生的事情。
例子：
- The accident happened at 7 o'clock in the morning.
(这次事故发生在早上7点。)
- I happened to see her at the supermarket yesterday.
(我昨天碰巧在超市看到了她。)
4.频率：happen较常见，可用于各种场合；happen to较少见，多用于口语和非正式场合。
例子：
- Such things happen often.
(这样的事情经常发生。)
- I happened to see him.
(我碰巧看到了他。)
5.语气：happen表示客观事实，中性语气；happen to表示主观感受，带有一定的偶然性和意外性。
例子：
- It happened to be the best decision I've ever made.
(这是我做过的最好的决定。)
- This kind of situation doesn't happen often.
(这种情况并不经常出现。)