I am facing these two sentences:
Dann geht er ins Wohnzimmer.
The word order is: Verb, subject, … Clearly, the sentence is not a question.
The other sentence is:
Er macht den Fernseher an.
The word order is: Subject, verb, …
I want to know:
You can hardly say what the more popular word order is because you change word order for different reasons. Sometimes grammar requires you to swap words, sometimes it's just for the purpose of emphasis.
However, the 'standard' word-order (what you'll learn in school first) is: S-P-O
Er geht ins Wohnzimmer.
Er macht den Fernseher an.
If you use main clauses only and wouldn't add any emphasis, you would use this word-order only.
Er geht ins Wohnzimmer. Er macht den Fernseher an. Er setzt sich hin. Er schaltet um. Er schläft ein.
This if, obviously, not very thrilling. You can use conjunctions like dann (an adverbial conjunction, btw) to create kind of connection between the actions. In this case, German grammar requires you to swap word-order to P-S-O.
Erst geht er ins Wohnzimmer. Dann macht er den Fernseher an. Anschließend setzt er sich hin. Nach einiger Zeit schalter er um. Letztlich schläft er ein.
Still, this wouldn't be quite sophisticated German, but it's a first step towards it.
Anyway, this is the reason why you're seeing a different word-order in your sentence. There's no 'more popular' word-order. You wouldn't use an adverbial conjunction in each and every sentence and even if you could still maintain the S-P-O word-order by moving that conjunction into the middle of the sentence.
Er geht erst ins Wohnzimmer. Er macht dann den Fernseher an.
The important things you have to remember:
Expanding on Em1’s answer I was bored and did a sentence order count in one random long post of an Austrian on this site known for his long and detailed answers. I only counted main clauses.
The ‘standard’ subject-verb sentence, where subject is first, verb is second and I didn’t care what followed then, (SV_) occurred 23 times (including two sub-headers).
Pulling an adverbial, an apposition, or something else into first position, with verb following second (AdvVS_) occurred 8 times.
An object was pulled before the verb (OVS) 3 times.
A relative or otherwise subordinate clause occupied the first position two times, forcing ScVSO.
On one occasion did I notice VSO outside of a question, although that is probably debateably truly a conditional subordinate clause. (Wird […], kann […] or something like that.)
Finally, having a conjunction in zero-position (ConjSV_) happened a mere 4 times.
So excluding the VSO special case, in 58 % of all cases the word order is subject — verb — object as one is taught at school and one expects it. One could add the 10 % of cases where SVO is preceded by a conjunction.
In 32 % of cases, the word order is something — verb — subject, i.e. something was pulled into first position to emphasise it, deffering the subject to a position after the verb. (Some sentences didn’t include a subject in this construction. However I didn’t include that into my considerations because I didn’t count the expletive sometimes present in SVO sentences either.)
But it’s much better to say that 90 % of all main clauses had the verb in second position, no matter what preceded it. The 10 % were the aforementioned zero-position conjunctions (Aber es gibt Ausnahmen).
tl;dr: 68 % of the sentences in the sample had SVO. But 90 % followed V2 (meaning it doesn't matter what precedes the verb, as long as there is something). Please now decide what the most general word order is.
