I usually get confused between those words, when I want to use them while speaking. They are very close to each other, yet they have completely different meaning (at least from my mother tongue perspective) I’m sure that if we dig into the roots of these words, we will find something common either from a historical perspective or from meaning by a thorough explanation. Especially when it comes to people who are their English is as a second language. Since language is not only letters and words, instead it’s a pattern of thinking.
Do they share some nitty-gritty details behind the scenes?
Answerdepress would be to analyze what distinguishes them: their prefixes. Unfortunately, in this case the Latin prefixes ob- and de- basically mean ‘down’ and sub- means ‘beneath, under’. All three words with these prefixes still bear some vestige of pressing down, but the distinction comes in who or what is doing the pressing and who or what is getting pressed down.
Native speakers have learned to use these words without the slightest idea of their etymology, mostly by reading and hearing them used enough so that both meaning and usage become a habit.
You can shortcut that process by consulting a dictionary and paying attention to the example sentences, then doing a query for the particular word in Google Books to see how it is used by educated native speakers.
If you still have difficulty remembering the difference, make up a sentence like:
The oppressed people revolted, but the king suppressed the revolt, which depressed the economy — and the people.