a) He explained it so that we could (very) easily understand it.
b) He explained it so that we could understand it (very) easily.
c) He explained it so that it was (very) easy for us to understand.
d) He explained it so that it was (very) easy to understand.
"He explained it easily" would have a different meaning from Sentences a)-d). Sentences a)-d) mean that probably he explained something clearly and simply and because of the clarity and simplicity, we – not very well versed in his subject – were able to understand it.
If you used "He explained it very easily..." the sentence would be like this:
e) He explained it so easily that his professor was amazed and gave him an A+ for the course.
f) He explained it very easily, so the professor knew that he was familiar with the topic.