糟糕, 对不起. 我忘（it should be changed into "忘了”）我已经约过朋友吃饭。（it's better to add"所以"here, but if u omit it, this sentence is still ok）我不能跟你们吃饭。 My comments: About past tense in Chinese In Chinese, we often use the word "了” which follows the verb. to express past tense in English. Eg1. I forgot I had invited another guy to dinner. 我忘了我已经约过朋友吃饭。 Eg2. I play basketball. 我打篮球。 Eg3. Yesterday, I played basketball. 昨天，我打了篮球。 Btw, it’s very great of u , u even know to use the word “已经” to express past perfect tense! ：）

the first is"这个对吗" not “这个对马” and then the sentence is a little wrong and a little impolite ，you should say “糟糕了，我已经约了朋友吃饭，今天不能和你们一齐吃饭，对不起”

糟糕，对不起啊，我忘了我已经约了朋友一起吃饭了，今天不能跟你们一起吃了。

不好意思，我忘记我已经事先和朋友约好要吃饭了，但是，我们可以下一次去啊~！

糟糕, 我忘了我已经约过朋友去吃饭。我不能跟你们去吃饭了，对不起.！