I don't actually know how to translate "it was fun" into Chinese, as it's all about the context! For 'I had fun tonight', you can say something like "我今天晚上过得很开心“。When people hear this, they would assume you are saying this by the end of the night, there is no need to emphasise the past tense. It was cold there. -那边曾经很冷 if you really want to emphasise it was in the past, though this is more like 'it used to be cold there'... But again there is nothing wrong if you just say '那边很冷‘ when you are referring to a trip you went on. Hope this helps! Sorry if this is not totally clear...

the soup is cold, you'd better heat it up. 汤冷了，你最好加热一下。 it's cold outside, you'd better put up your coat. 外面冷，你最好穿上外套。

了 is a helping verb~ A helping verb only helps verbs. The past tense of "S+V+P“ sentences in Chinese should be clarified 1:by the adverbial of time 2:by the main tense of the context/ by the language environment. 1: My mom is beautiful. 我妈妈很漂亮 My mom was beautiful. 我妈妈以前很漂亮。