The usual position for adverbs of frequency is before the main verb, "he often flies ..." and unless you're up to native-speaker standard it's advisable to stick with this.
It can sometimes go at the beginning or right at the end, "He flies to the capital quite often", or "He flies quite often". But we don't separate the verb "go" from its destination "to the capital".
It is very hard to make clear 'ę' here. Almost always it will sound something like czensto, częnsto or czełsto (this is a clear mistake). The sound 'ę' is not denasalised completely, only partially.
Please notice, that pronoutiation implied by rules from https://mowicpopolsku.com/polish-alphabet-pronunciation/#nasal-vowels is quite different, than pronoutiation according to https://pl.wiktionary.org/wiki/cz%C4%99sto. So this set of rules seems to be incomplete.
I have found probably good set of phonetical rules of polish language: http://free.of.pl/g/grzegorj/gram/pl/gram01.html. According to these rules:
- przed zwartą lub zwartoszczelinową zębową lub dziąsłową wymawia się n: kąt [kont], ręce [rence], pączek [ponczek]
This rule can be applied to “często” since ‘s’ is voiceless alveolar fricative consonant (spółgłoska szczelinowa dziąsłowa bezdźwięczna). Therefore "czensto" might be the only correct pronunciation.
Please don't look for general information and try to extrapolate specific rules from it. That doesn't work. And generally, don't create any more posts unless you've researched the topic thoroughly. The moderation team is constantly occupied with neutralising your misleading comments.
Przed spółgłoskami szczelinowymi: [z], [s], [ż], [ś], [sz], [w], [f], [ch] wymowa [ą], [ę] nie ulega zmianie.