Below is the block of code to use If/Else condition under LWC UI (.html file). Condition variable you can defined under .js file.
<!-- If Condition-->
<template if:true={your_Variable_defined_in_JS}>
<!-- Put your UI logic here. -->
</template>
<!-- Else Condition-->
<template if:false={your_Variable_defined_in_JS}>
<!-- Put your UI logic here. -->
</template>
