该句子为“主句＋连词＋宾语从句（主语＋谓语＋……）”句式。
1、先看整个句子：I为主语,frogot为谓语，后面整个句子是宾语从句。
2、在看宾语从句句子的成分，连接词＋名词＋主语＋谓语。I为主语,be supposed to为谓语，go to which room 为宾语。在宾语从句中将连接词which放置在了前面。which room 是介词to的宾语
一般在句子中，无论主句是陈述句还是疑问句，宾语从句都必须使用陈述语序，即“主句+连词+宾语从句(主语+谓语+……)”句式。根据连接词在从句中所担任的不同成分，可分为以下四种：
1)连接词+谓语。连接词在从句中作主语。常见的连接词有： who，what，which等。如：
Could you tell me who knows the answer，please?你能告诉我谁知道答案吗?
The small children don't know what is in their stockings.这些小孩子不知道袜子里有什么东西.
2)连接词+名词+谓语。连接词在从句中作主语的定语。常见的连接词有：whose，what，which，how many，how much等。如：
He asked whose handwriting was the best in our class.他问我们班上谁的书法最好。
The teacher asked us how many people there were in the room.老师问我们房间里有多少人。
3)连接词+主语+谓语。连接词在从句中作宾语、状语或表语。常见的连接词有：who(m)，what，which, how many, how much, when, why, how, where, if /whether(在句中不充当任何成分)。如：
He hasn't decided if he'll go on a trip to Wuxi.他还没决定是否去无锡旅行。
Could you tell me what I should do with the money ?你能告诉我我如何处理这笔钱吗?
4)连接词+名词+主语+谓语。连接词在从句中作宾语或表语的定语。常见的连接词有：what，which，how many，how much，how等。如：
Do you know which class he is in ?你知道他在哪个班吗?
She asked me if I knew whose pen it was.她问我是否知道这是谁的钢笔
（本句中就是第四种形式）
3、suppose v. 推断；料想而be supposed 可看做被动语态，某人被料想做某事，潜意思就是希望某人做某事，也就是某人应该做某事。
追问我不明白的是后面的to go to是动词不定式吗？
I forgot which room I was supposed to go to.
我忘了我应该去哪个房间了。
which引导宾语从句追问
从句成分？
追答从句的成分如下，
I是主语，was supposwd to go to 是谓语，which room是宾语。
其实这句话也可以写作：
I forgot the room which I was supposed to go to.
为什么to go to能作谓语
追答严格来说，to go to the room应该做谓语be supposed的谓语补足语
I 主语
forgot 谓语
which room I was supposed to go to 是宾语从句。
宾从中:
I 主语
was supposed to go to 谓语
which room 是介词to的宾语
如还有疑问，欢迎追问，有问必答！
追问to go to为什么能作谓语？