1."I was/am going to go out when you arrive..."means 你来的时候，我本来/正要出去了。 2."be going to do something" is just like the changing aspect of an action in Chinese,not the Prestent/past countinuous Tenses. In Chinese ,when an action is going to change or to take place relatively soon,we use the pattern "要....了"to describe it.And this pattern mainly indicates that an action is going to take place in the future,but it can also show that an action was going to take place sometime in the past. e.g.昨天飞机要起飞了，忽然下雨了。(The plane was setting out yesterday when it rained suddenly .) e.g.上星期我要去旅行了，碰巧生病了。(I was going to travel when I was sick accidentally last week.) e.g.快点，客人就要到了。(Hurry up,the guests are going to arrive.) 3.In Chinese,the aspect and the time of an action are not entirely expressed in one grammatical form.The state of an action has something to do with the time,but does not indicate the time.Thus adverbs and aspectual particles are often used to denote the different states of an action while the time of an action is mainly expressed by nouns ,adverbs or phrases denoting time.

i am going to do sth 我正准备要do sth I was going to go out when you arrive 你到这的时候我本来是要出去的

I am going to go out when you arrive 你来的时候， 我正准备出去呢. or 我马上要出门的时候， 你来了 I was going to go out when you arrive 你来的时候， 我本打算出去的。 or 我计划出门的， 结果你来了 The difference could be slim or obvious by how you say that . but definitely could seperate the two tence if you want . Btw , i am going to go out when you arrive, this sentence seems not right in english . well, anyway, hope i make the chinese part clearly enough

I am going to go out when you arrive... translate: 当你到的时候，我正要/刚好要出去 I was going to go out when you arrive... translate: 我正要出去时，你到达 My opinion: In fact, no difference between these two sentence..Chinese with english is different.English has many tenses,but Chinese more use grammar

正要，打算。你来的时候我正打算出门呢。中文对于时态在表达方式上没有太多要求，如无上下文参照，可能无法断定是现在时还是过去时。