* His refusal to testify was tantamount to admitting guilt. (This is a common and effective use, showing the refusal is essentially equivalent to an admission.)
* Ignoring the safety warnings was tantamount to negligence. (Shows the inaction is the same as being negligent.)
* Leaving the child unsupervised for hours is tantamount to child endangerment. (Highlights the equivalence of the action and the serious crime.)
* Spending beyond your means is tantamount to financial ruin, if not already. (Shows a consequence equivalent to ruin).
* The constant criticism she received was tantamount to emotional abuse. (This illustrates that the criticism is as damaging as abuse.)
Notice that "tantamount" is always followed by "to" and then a noun or noun phrase that represents the equivalent action or condition.