An infinitive can act as a neuter noun: the subject of est, or the object or complement of verbs like possum, desino and memini; translate "to do" or "doing".
As subject: an infinitive with est and an adjective or noun, "to do this is …" or "doing this is …". An adjective agreeing with it is neuter singular: nihil est grave "is not grievous at all", ipsum mori "dying itself".
As object or complement: after possum, debeo, volo, soleo, desino, memini/memento and similar verbs, the infinitive completes the meaning: praedicere memento "remember to say beforehand".
The subject infinitive may come after est, and a genitive may say whose task it is: Facultatis rationalis est, considerare "it is the task of the reasoning faculty to consider" (see the genitive of characteristic).
The infinitive has no cases. For "of doing", "by doing" or "for doing" Latin uses the gerund instead (agendi, agendo, ad agendum).