求简洁地讲清楚原因--------（查找）the new wors in dictionary if you don't

求简洁地讲清楚原因
--------（查找）the new wors in dictionary if you don't know them.是填look up还是looking up?我知道if后面是从句,但是前面的这个句子中没有动词,可不可以填look up?这个又不像祈使句