Fancy the goddamn network. I wonder whether it's the problem of my ISP or italki's server that often caused automatical logout of my account.

just removed "le" will make it perfect itself. :) not all the cases descripe past tenses with "le"

Yes,that's right . . "zuo tian" means "yesterday"."jin tian" means "today" . "ming tian" means "tomorrow" .But I think it's better to say "zuo tian xing qi liu ." In Chinese ,"le" has a little sense of "already".

It would be right in saying "zuo tian xing qi liu" without "le" if you just want to say a fact. But if you want to express doubts, surprise or something emotional according to your context, "le"can be used as an exclamation with rising or falling tone in the end. It can mean "?", "already", "has been", etc. Hope others will add something I missed.

"zuotian xingqi liu le" sounds a bit weird to me. would be better to remove "le". I can't explain why it is so, grammar and me don't go. :P I just know the sound,if it sounds right,then it's right.