If the adverb ‘certain’ can be used as an adverb by adding an ‘ly’ (certainly), then it’s synonym ‘sure’ can surely be made into an adverb by adding ‘ly’ without a change of meaning.
No, it can’t. It ‘surely’ is a different word to ‘certainly’, and not the adverb of ‘sure’.
This mistake by students is so common and misusing a word like this could drag your lexical resource score as low as 5 (depending on how well you manage other vocab in the essay).
Naturally, the internet and the user of English as a second language look for rules and neatness that they can file away and set in concrete. But for a native of any language; languages is culture capable of subtle meanings, sarcasm, irony and other forms of humour and tact, that come from living the language and not learning it. This miss use of ‘surely’ sets to simplify and dumb down English.
Here is how ‘surely and ‘certainly’ are not the same to native speakers.
Adjective forms
“I am sure about my decision.”=: “I am certain about my decision.”
Adverbs
“I will certainly succeed.” (strong probability/fact) is not the same as
“I will surely succeed.” (a hope to succeed/ determination, it could be a doubtful wish depending on the tone)
Adjective:
“That is a certain outcome.”= “That is a sure outcome.”
Adverb:
“That will certainly happen.” (strong probability)
“That will surely happen.” (a hope or deduction, depending on the context it could be ironic, or sarcastic.)
Different meaning 1. ‘thoroughly‘ or ‘with precision‘ or ‘with certainty‘
When it modifies a verb of human action directly such as ‘he speaks surely,’ it means ‘thoroughly’ or ‘with precision. Consider these differences:
“He speaks certainly and confidently.” – “certainly” doesn’t work here. The speaker means ‘with certainty’ but certainly is probably (he speaks definitely or he obviously speaks). As a probability this is an unnatural sentence.
“He speaks surely and confidently.” – ‘surely’ works here meaning ‘thoroughly’ or ‘with precision’.

Different meaning 2: to check surprising information colloquially.
“Certainly, you can’t be serious.” (agreeing with strong probability)
“Surely you can’t be serious.” (contradicting the listeners agreement.)
Sample: Classic comedy scene from Airplane! (1980):
Dr. Rumack: “Surely you can’t be serious.”
Ted Striker: “I am serious… and don’t call me Shirley.”
Different meaning 3: When used as a cohesive device in essays.
In this abstract it is used well to
Women have played a crucial role in scientific advancements throughout history, yet their contributions have often been overlooked or undervalued. Surely, the progress of science would not be where it is today without the groundbreaking discoveries and innovations brought forth by female scientists.
In written writing such as essay, it is used well, as the writer uses ‘surely’ to question her previous statement and develop a nuanced argument.
In this essay abstract it is misused.
Leaders have always been taken in high esteem and regarded as ideals by billions of followers. Surely, people donning the role of the leaders have traits that define intelligence, and it is the intelligence that makes them naturally fit for leadership.
In this essay abstract the writer meant ‘certainly’ as he was not questioning the previous statement but reconfirming it.
Hence, ‘certainly’ as a cohesive device in writing reconfirms the previous statement, while ‘surely’ questions it.
What should an exam candidate do?
Avoid ‘surely’ altogether if what I have been explaining is in anyway surprising to you. Use ‘thoroughly,’ ‘with precision,’ or paraphrase.
‘Surely’ isn’t on any list for I.E.L.T.S. writing examiners, there are no list. Good language is specific to the topic, and the candidates position on the topic. But just keep in mind that if you misuse ‘surely’ and you are marked by a native speaker it will drag down your lexical resource. They might also decide you’ve used a dodgy template rather than wrote the essay yourself, and this will further affect your score.
On the other hand, if you laughed when you first say the movie Aeroplane, or already use it to colloquially question or tease your friends and colleagues this is the kind of a nuance that native examiners will note as a 9
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