Undermine and underline are easily confused words.
The spell-check application of most word processing software programs would not catch a slip-up of these two words. Spell-check is looking for words that aren’t in its dictionary, and words that resemble words in its dictionary but are possibly spelled wrong. Spell-check isn’t perfect. It doesn’t know and can’t guess what word you wanted or what word you meant, it can only judge the words on the page. If you used words that are all spelled correctly, it gives you a pass anyway.
Autocorrect suggests words that start with the same letters. It’s suggesting what word you may want to save time, but quite often, its suggestions are pretty off base. They don’t help you out, but they do make you laugh.
Undermine (pronounced “un-durr-mine”) is a verb. It means when a person behave towards their superiors in a disrespectful way, as if those superiors’ judgement or authority weren’t worth listening to or obeying for some reason. This might involve going to the superior’s boss.
Underline (pronounced “un-durr-lye-nn”) is a verb. It means to draw a line under text or other important content on paper (or virtual paper). It’s another way of saying “underscore.”
Command U is a typical keyboard shortcut for underlining selected text in word processing software.
In speech, underlining can be accomplished through adding emphasis or stress to certain words.
The following story uses both words correctly:
Ungus was disappointed when his cousin Ursula underlined the need for more public school funding in her speech. The crowd cheered. There were calls for her to run for office.
His re-election campaign was going rough. He felt her growing reputation was beginning to overshadow and undermine his.