第二、三句谓语为现在完成时，表示下雨开始于说话时之前。start后接to do和doing是没有差别的，二个句子意义相同。如果说有差别，就是如果starting doing的连接说起来拗口，需要改为starting to do. 除此之外几乎是没有差别的。

第一句用现在进行时，表示即将、马上要下雨了。这不是现在进行时表示将来。现在进行时表示将来是计划中的将来事件。而下雨并不能计划，不可能用现在进行时表示将来。

我同意my153老师的观点

It's starting to rain. 

我曾向多个外籍老师考证过，回答是“雨已经下了，但不明显，也许零星能感觉到”,这个逻辑完全成立，因为字面意思就是处在starting阶段。

关于下还是没下雨的理解：

（1）（2007年有人问过）

1. It Has Started To Rain.

It has started to rain.
It's starting to rain.
It has started raining.
Can someone explain the differences between the above sentences? I don't see any difference.

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You look out the window and see those VERY FIRST drops or the pavement just starting to get wet - it's starting to rain.

You look out the window and it's raining, and the pavement is just about all wet, but it's not coming down hard (yet!) - it's started to rain.
And in truth, you can use either one in either situation without sounding too weird.

链接：https://www.englishforums.com/English/Rain/vqbmn/post.htm

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（2）（2013有人问过）

But I also found in a book that describes
1. "It is starting to rain" is correct
2. "It starts to rain" is incorrect
in this example.
Both of them seem to be the same to me, what are the difference between "start to" and "is starting to"?

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BE： You can say "It is starting to rain" in answer to that question because it's describing something which is actually taking place at the time. A context in which you could use the simple present would be something like: "Every time I go out without a coat, it always starts to rain".

链接： https://forum.wordreference.com/threads/start-to-vs-is-starting-to.2750636/

再补充这里时态的理解：

'It's raining" : Present Continues indicates that the rain started prior to NOW, the moment of speaking; continues as an event, but has a foreseeable END = "It's stopped raining."

So, if it's already started, how can you say, at the same moment NOW, that it is starting raining ???

'start' : begin or be reckoned from a particular point in time or space.

Language is such fun!

链接：https://forum.wordreference.com/threads/starting-raining-to-rain.1819915/

语言要跟老外学，跟实践学，不要仅从书本学，

这个我似乎说过很多次了，某些人对书的理解层次可能会与实际大相径庭。

补充一下：

用完成时，说明 雨是既下未下，但处于突破那“下”的临界的状态

---即动作/事件已经开始，（未必就是事件的完成，---比如动作的结束，状态的开始）

用进行时，那动作注定在之前某个时候已经开始(趋向性动词表将来算是个例外)。

（请勿追问）

(1) It's starting to rain. 

(2) It's started to rain

(3) It's started raining.

第一句不是表示还没有下雨、即将下雨的意思，而是表示：“（正在）开始下雨了。”