是表语从句。
1. It seems that he has forgot our appointment.
seem是系动词，所以that从句是表语，即表语从句。——系动词后的成分，无论是单词、短语还是句子，都是表语。
2. It seems obvious that he has forgot our appointment.
seem是系动词，所以obvious是表语。
本句的It是形式主语，指代that从句，所以that从句是主语从句。既然是主语从句，所以这个句子也可以是：
That he has forgot our appointment seems obvious.
例：He appears to be very tired. 他显得很疲惫。
例：She is a teacher. 她是一名老师。
例：It became colder in the evening. 晚上天气变得更冷了。
例：The handle came loose. 把手松了。
例：This letter constitutes an official apology. 这封信构成正式道歉。
例：Five plus five equals ten. 五加五等于十。
例：He fell silent after hearing the news. 听完消息后他变得沉默。
例：The fabric feels soft. 这块布料摸起来很柔软。
例：She got angry at his words. 她听了他的话变得生气了。
例：The milk went sour. 牛奶变酸了。
例：He grew impatient waiting for the bus. 等公交时他变得不耐烦了。
例：Please keep quiet in the library. 在图书馆请保持安静。
例：You look beautiful tonight. 你今晚看起来很美。
例：She will make a great doctor. 她会成为一名好医生。
例：This room measures 5 meters across. 这个房间宽五米。
例：The task proved more difficult than expected. 结果这项任务比预想的更难。
例：He remained calm during the crisis. 危机期间他保持冷静。
例：The river ran dry in summer. 这条河夏天变干了。
例：She seems happy today. 她今天显得很开心。
例：The flowers smell sweet. 这些花闻起来很香。
例：Your idea sounds great. 你的主意听起来很棒。
例：He stood firm in his decision. 他坚持自己的决定。
例：Try to stay calm. 尽量保持冷静。
例：The soup tastes delicious. 汤尝起来很美味。
例：His face turned red. 他的脸变红了。
例：This package weighs 2 kilograms. 这个包裹重两公斤。
