The phrase "to explain it further" is correct and usable in written English. You use it when you want to explain a topic in more detail or to provide additional information. For example: "The process of photosynthesis is complicated, so I'll explain it further."

"to explain further" is correct and can be used in written English. You can use it to introduce an additional explanation to a concept or idea you have already discussed.

It is used when you want to add additional details or examples to clarify an idea. For example, "The new policy has caused confusion, so please further explain what is expected of each department." It did not further explain either standard. Or I can further explain my reasoning.

"to further explain" is correct and usable in written English. You can use it as a way of introducing additional explanation or clarification when you have already provided some information on a topic. For example, "The atmosphere in the school was tense.