You can explain like that. One of le's usage is to indicate the action finished. In fact, we can't simply say it's a sign of past tense.

You can use it in the second sentence because dinner will be already finished when you will go to cinema. So even if you will go tomorrow which is in the future, the dinner will be in the past in relation to the moment when you will go to cinema. So "le" is expressing the relation between having dinner and going to cinema. Btw. what movie you will see? ;-)

Some people will explain that 'le' indicates past tense. If it's true, why can we use it in the second sentence? It's future.

I am sorry, firehorse. I am afraid I can't catch you. By the way, thanks for you picked out my fault. I have corrected it.

Here are some sentences: 1. Wǒ chī le wǎnfàn。(I have had my dinner.) 我 吃 了 晚 饭 。 2. Míngtiān wǒ chī le wǎnfàn qù kàn diànyǐng。 明 天 我 吃 了 晚 饭 去 看 电 影 。 (Tomorrow I will go to the cinema, after my dinner.) Do you know how to use 了(le) in Chinese correctly? Give me your answer or you also can give me your sentences。Let's see if they are expressed in a correct way. Good luck!