Hello 

I think your understanding of these 2 is perfectly correct!!

For 了, I think I treat it as pass tense.  Just like the 'agreed' is a pass tense. Chinese verb don't change forms for tense, instead we add different word to show different tenses.XD  If you want to use already, I usually prefer 已经 in Chinese. 

The second on is just like what you think. just like English has many different ways to say a same sentence. only you will have to  change many words in English but this sentence just change one word in ChineseXD 

Ruby 

Hi Peter,

In the context which you’ve given, there were only 2 options available - to call it off or to proceed as planned. Hence, the “要” is being used to emphasize this: “we have already agreed on going so we have to go regardless of the weather conditions”.

Essentially these 3 sentences mean the same:

1) 我们说好了要一起去的; 2) 我们说好了一起去的; 3) 我们说要一起去的。

However, ”我知到。 但是。。” which means “I know but...” tells you that the speaker wishes to emphasize that he is well aware of the situation but he wishes to proceed. Hence he added “要” to emphasize that the outing “must” take place.

”了” is usually used to express the completion of a verb:

I’ve already eaten: 我吃饱了

I’ve already read many articles: 我阅读了大量的文章

Hope this helps!