look forward to 后只能接动名词吗?有没有接动词原形的时候?

没有,只能接动名词.因为这里的to 是介词,介词后面只能接名词,代词或动名词.如果接动词原形的话,那就是动词不定式了.下面是to 的用法总结,请自行参看以下：
第一、作介词：
1．表示方向,译“朝、向、到”的意思,例如：I often go to school by bus.
2．表示时间,译“到……为止”的意思,例如：I will stay here to mid-night.
3．表示间接关系,译“给……”的意思,例如：I will write to you soon.
4．表示目的,译“为了”的意思,例如：Let’s drink to your health.
5．表示关联关系,译“至于、对于”的意思,例如：He is so kind to you.
6．表示结果、效果,译“致使”的意思,例如：To our surprise,he didn’t come.
7．表示所属,译“……的”的意思,例如：This is a key to my bike.
此类用法主要还有以下这些词：answer,reply,memorial（纪念碑）,bridge,
monument（纪念碑）,entrance,traitor（叛徒）,note,burden,visit,explanation等.
第二、作动词不定式标志,接动词原形,充当主语、宾语、表语、定语、状语宾语补足语这六种句子成分,例如：
1．To learn English well is not easy.
2．They like to play football.
3．My job is to teach you English.
4．Do you have something to tell me?
5．They often go to have a walk after dinner.
6．Our teacher asked us to recite the new text.
第三、用于固定结构,例如：
1．I have to go to have a check of my head.
2．They used to swim in this river in summer.
3．We are looking forward to seeing our new school.
上述to被看作介词,后接动词-ing形式还有以下这些词：
pay attention to,be/become/get used to,devote sth/oneself to,get down to,be due to,see to,feel equal to（能信任）,in addition to,admit to,be familiar to,find one’s way to（设法达到）,give way to（对……让步）,give one’s mind to(专心于),keep to(坚持),be open to(愿意接受),look up to(尊敬),lead to,object to/be opposed to(反对),stick to,stand up to(勇敢面对),be sentence d to(被判刑)等.