The aim of this paper is to investigate syntactic features which are carried by the finite complementizer þæt>þat>that and the non-finite complementizer for from diachronic perspective. Given that þæt/þat wasn't followed by a focalized or topicalized element, but permitted wh-word+þæt/þat sequence from an embedded clause, þæt/þat had the finiteness feature of φ-features and a tense feature that was agreed with the finite feature of a finite verb in OE and EME. On the other hand, considering that that has been followed by a focalized or topicalized element in the embedded clause and φ-features and a tense feature in C has been inherited to T from LME onward PDE, that came to carry a declarative force feature rather than a finiteness feature from that time on. Meanwhile, the prepositional complementizer for began to be added to the to+N/V+enne to strengthen the grammatical function of to from EME, and this phrase has developed into an infinitival verb phrase for+to+V+(O). As inflections were lost, V-2 was reanalyzed, and OV was reordered as VO, the for+to+V+(O) sequence was changed into an infinitival verb clause and ultimately transformed into for+Osubj+to+V in LME. With such restructuring of an infinitival verb clause, a non-finite future feature which was inherently carried by for came to be inherited to the infinitive marker to that started denoting an unrealized future tense ever since.