Simply
Merely, Only
adverb
If you're moving towards (に) simple, you're saying simply, merely, or only.
たんに
If you know the readings of your kanji you'll know how to read this as well.
コウイチは単に大変な努力家なだけですよ。
It's simply because Koichi is a very hard worker.
「トーフグは、私にとっては単にお金をかせぐためのものではないんです。トーフグこそが私の人生なんです。」と、コウイチは言った。
Koichi said, "To me, Tofugu is not merely for earning money. It's my life."
剣道では、単に相手を倒すことだけではなく、自らの心を鍛えることも重要です。
In kendo, it’s important not simply to defeat your opponent but also to train your own mind.