Because "probably" translates to "probablement" in French (4 syllables), to make it quicker we broadened a little bit the meaning of "sûrement". Depending on how much emphasis we put in the word, the degree of certainty can vary quite a lot and definitely embraces some situations where you would rather use "probably".
Interesting point. This sounds very parental - a classic loaded question. There is uncertainty, but you know up front what the 'right' answer is. Lots more communication going on than just probability.
I have noticed when watching 7de Laan, the local subtitled Afrikaans soap, that 'seker' (is jy seker? are you certain/sure?) is often used to mean 'probably' .... I wonder if there aren't more examples of this in other languages.
Guys? Any other offers?
He is certainly going to come. (Position of 'certainly'.)
In the UK, we would probably say 'definitely' rather than 'certainly'. No good reason - it's just what we would say. :)
PS A: Is he going to come? B: Yep. A: Definitely? B: Absolutely. ... and I still haven't used 'certainly'. :)
One admittedly specific case where this could work would be if there was a second action that WASN'T certain. You can use this form to contrast the two. This is mostly used in spoken English, since it needs an certain tone of voice to make sense.
Example: "He is going to certainly come, though whether he will stay, we don't know."
That said, if you're a non-native speaker then you're better off sticking with "He is certainly going to come", as that always makes sense. This is a nuanced example that is used on rare occasions.
Split infinitive construct is technically incorrect or at least informal. Your example should still be, "He is certainly going to come, though whether..." or, "He certainly is going to come, though whether..."
If someone was going to come while being certain, it should be, "He is going to come certainly."
In this context, the split-infinitive construction is awkward, no question. But in general, splitting an infinitive is not wrong—that's an old English teacher's myth (derived from the fact that in many non-English languages, infinitives are single words and thus can't be split the way they can in English)—right up there with its supposedly being wrong to end a sentence with a preposition. Changing such constructions to reflect better phrasing is great, but doing so mechanistically just to fix a supposed error is overstepping.
I notice I got down-voted for this, and my only response is to quote (i think) Raymond Chandler's reply to a literary critic:
"When I split an infinitive I'm going to goddam split it so it stays split"
This is a personal bug-bear of mine - split infinitives are NOT a grammatical error and, except for during a brief period of academic debate in the 19th century, never have been.
I put "he is surely going to come" and it was marked correct. Then, however, Duo gave me another correct solution as "He is probably going to come.", which means something entirely different. Is this really the case that this french statement can be translated in very different ways in English?
One of the definitions offered by Dictionary.com for surely:
(in emphatic utterances that are not necessarily sustained by fact) assuredly: Surely you are mistaken.
When surely is used this way there is a tinge of uncertainty. It is less than certainly because it is unsupported by fact, but more than maybe. When the speaker says certainly he is saying he has very good reasons to support his belief. When he says surely he is saying that he has great confidence in his statement. Surely is the more subjective of the two terms. There is a slight difference, for which reason Duo will accept probably even though that is less positive than surely in its import.
In English, surely is different but not entirely different from probably. Surely is similar to but not identical to certainly.
F.W.I.W. Larousse treats sûrement as meaning certainly and does not seem to favor Duo's use of probably as a likely definition. Larousse doesn't mention probably at all. Neither do a couple of other less authoritative French/English dictionaries.
You are correct to be concerned about the definitions Duo attributes to sûrement but on less solid ground with respect to the English usage.
Va puts the sentence into the future tense - he surely IS GOING to come. In fact, I'm not sure it would make sense without it, as venir is the infinitive, and you would need to conjugate it.
If you left out "va" you would need to write it as "il vient surement" - "he surely is coming".
You could argue that this doesn't substantially change the meaning of the sentence.
I see no problem with 'for sure'. It's casual usage - not usually written, or used in formal situations, but it's OK. Go for it. (<- also casual)
PS When you wrote 'surly', did you have a particular man in mind? (You know it means grumpy, bad-tempered, right?)
Have a sunshine day. :)