I didn't speak to anyone.I speak to no one.=I speak to nobody吗?

对，
当表示数量时，英语一般有几种方式表示否定，
比如：There is not any milk in the bottle.
=There is no milk in the bottle.
形容人也是一样。

speak要改为spoke,no one=nobody基本对

对的。 不过后两句中，speak要改为spoke,过去式啊

speak要改为spoke,过去式啊

同意一楼
不过三者之间有些许不同.
I didn't speak to anyone.
语气较重,汉语可以理解为我跟谁也没说话.
I speak to no one.
设置一个场景有助于理解这个哨位差别：
Whom are you speaking to?
I speak to no one.I'm talking to myself.There is nobody here.
I speak to nobody.
There are so many people,whom did you speak to?
I spoke to nobody.
平时用时三者意思相当.

对的.