“不是认真地”常见的英文表达有：
not seriously：这是最直接、常用的表达。例如：He didn't take it not seriously; he was just pretending.（他并非不认真对待，他只是在装样子。不过从更自然语境理解，这里想表达他其实是认真的，只是装作不认真，更自然的说法可能是“He was actually serious; he was just pretending not to be.” 但若按原句结构翻译，“not seriously”放在这里逻辑稍显奇怪，此例仅为展示“not seriously”用法。更合适的例子：I didn't mean it not seriously.（我并非不认真这么说。 ）
not in earnest：“in earnest”表示“认真地；严肃地”，“not in earnest”即“不是认真地；不严肃地”。例如：He was just joking, not in earnest.（他只是在开玩笑，不是认真的。）