This pattern is used to say that something is not enough to solve a problem or avoid punishment. It implies that a more severe outcome is inevitable. For example, if you break something, just apologizing won't be enough; you'll have to pay for it. The nuance is that the speaker believes the situation is serious and cannot be brushed off with a simple action or excuse.

The phrase では済まない is the negative of で済む, which means "to end with" or "to get away with." So では済まない literally means "it won't end with just ~." It often carries a tone of warning or criticism. It is used in both spoken and written Japanese, but is more common in formal or serious contexts.

When using with a noun, you attach では済まない directly to the noun (e.g., 謝罪では済まない). With a verb, use the te-form followed by は済まない (e.g., 謝っては済まない) or the nominalized form with の (e.g., 謝ったのでは済まない). The te-form version is slightly more conversational, while のでは is a bit more formal and emphatic.