Print out Sub Categories based on Main Category ID

I was able to find the main category ID of a single post

please start by posting the code – any suggestion might depend on what you have so far.

Thise code prints out all of the main categories of a single post:

<?php
foreach(get_categories('parent=0&orderby=id') as $cat) {
   echo $cat->term_id.' '.$cat->name.'<br/>';
}
?>

I want to be able to print out the sub categories underneath that main category on the same line.

your code will show the top categories of your site – not of the individual post.

try:

<?php
foreach(get_the_category($post->ID) as $cat) {
   if(!$cat->parent) : //check if top cat
   echo $cat->term_id.' '.$cat->name.' (top cat)<br/>'; //top cat
   $sub_cats = get_categories('parent='.$cat->term_id);
   if($sub_cats) foreach($sub_cats as $sub_cat) {
   echo $sub_cat->term_id.' '.$sub_cat->name.'<br/>'; //sub cat(s)
   }
   endif;
}
?>

https://codex.www.ads-software.com/Function_Reference/get_the_category
https://codex.www.ads-software.com/Function_Reference/get_categories

Worked perfect thanks!