ernst vs ernsthaft
Could someone please tell me the difference and uses of those two words as they both translate to serious or seriously
That answer is incorrect. "ernsthaft" can be an ajective, not just an adverb. See, for example
Hello drvdw, I had to look up it. (Your post unsettled me.) "Ernsthaft" is an adjective, but you can also use it as an adverb.
Here is a good link, where you can read it:
(Sorry, I didn't see I replied to OP's repost at first, so I guess I'm repeating things that the already given links say...)
Often "ernst" is used as an adjective to describe e.g. a person ("Er ist ernst", "Die Lage ist ernst = The situation is serious/critical"), while "ernsthaft" is used as an adverb to describe an action:
"ernsthaft" as an adverb, "seriously": "Ich lerne ernsthaft Deutsch" ("I seriously learn German, I am serious about learning German"), "Willst du ernsthaft da hinunterspringen?" ("Do you seriously want to jump down there?"), "Ernsthaft?!" ("Seriously?!").
You don't use "ernst" as an adverb, or anyway not normally: "Er arbeitet ernst an dem alten Auto." "He works seriously on the old car" = he (e.g. his face) is earnest/solemn/grave while he works; not: he works in a persistent/thorough way (which would be "ernsthaft").
So there I've already mentioned how "Er ist ein ernster Mensch" and "Er ist ein ernsthafter Mensch" are different:
"ein ernster Mensch" = an earnest/solemn/grave person who doesn't smile often
"ein ernsthafter Mensch" = a person who does things in a persistent/thorough way