could substitute each other generally。 but if go to the bottom ， there is a differ： 当。。。时 like when ， it attemps to express at the time XXX 当我蹲下时，发现厕所没纸了。 i found no paper in toilet when i got off my trousers;。 当。。。（的）时候 like while ， it shows during the time that XXX 当我想你的时候，泪水就流那样一直的流。 l am weeping while i am missing you 。

意思是一样的。 只是要注意，“当。。时候”通常要加“的”。说成“当。。的时候”。 而“当。。时”一定不能加“的”。 你可以把“当。。时”理解为“当。。的时候”的缩写。

几乎没区别 可以混用

2.当~~时。 当我发现留言时他已经走了。He's gone when i finding the leaving message. 3.当~~时候。 当你上车的时候，天忽然就暗下来了。 It's getting dark all of a sudden when you getting on the car. (in this sentence,the usage of "当~~时候" is nothing difference with "当~~时”） another one for “当~~时候” 请你忘掉我们的过去，当我离开的时候。 Please forget all of our past time when i'm leaving.(when "I" say this,"i'm leaving" is not yet to happen.and,you can't use "当~~~时" to instead of "当~~~时候") at the end,if you look carefully,you'll find there's always a "的" in front of “时候” but no "的" in front of "时". could think their difference by this way

they're a little different . 1.When you leave, don't forget to lock the door.你走的时候不要忘记锁门。（it's not a need to use "当" here。) 2.When you left, you forgot to lock the door!. 。你走的时候忘了锁门.(it's not a need to use "当" here too,and people know its tense by language environment. and there's a sign of it, is "忘了”,this part showed you it's past tense, and other parts of this sentence couldn't be such signs.) and,as you asked,i'll give some examples.at first,the two sentences you've given above are not accord with "当~~时/时候". i think you should makeout the two forms by understand one word---当时。do you know this word? 当时 means "at the time"/"at that moment",so it's easy to know that "当时” is a word wich accord with past continuous tense. then ""当~~时/时候" are derivational forms from it (当时）。they are apply to past continuous tense or past tense with a language environment of past continuous tense.. just the "~~" (between "当" and "时/时候" ) is a specific to the moment.（there's only one exception is "当~~时候” is also suitable for future tense<or language environment future tense> examples: 1.当时。 ------what were you doing at 21:00 yesterday evening?(昨天晚上九点的时候你在干什么？） ------i was reading a newspaper.(当时我在看报纸） (i didn't use "当时”<at that moment> here in english ,it doesn't means it's not a need in chinese. actually it's language environment, and i showed you tenses in English). such a conversation translate into chinese ,better with "当时" though there's no "at that moment" in english,"