Hey guys! How's it going?
I've just come across these two: doubtless and undoubtedly. They are synonyms. This 'doubtless' is a cheater which comes in disguise as an adjective, but it's an adverb.
Is either of them more formal or common in day to day English?

Thanks so much folks!
Hope you're doing well 🙂

Question

Hey guys! How's it going? 
I've just come across these two: doubtless and undoubtedly. They are synonyms. This 'doubtless' is a cheater which comes in disguise as an adjective, but it's an adverb. 
Is either of them more formal or common in day to day English?

Thanks so much folks!
Hope you're doing well 🙂

Answer

Oscar, I haven't heard the word doubtless but have commonly heard undoubtedly.  Of course this could vary by region.

Phrases that are common: 
Without a doubt
Beyond a doubt
Beyond a shadow of a doubt
That (something) calls into doubt

Answer

Let's not forget, "indubitably" :D

Answer

I'd say doubtless is pretty common (in the UK) - I certainly use it, though the Ngram viewer shows it is a bit less common than undoubtedly.