"Were you there?"
I was wondering if the forms with the -s ending are considered standard Czech? In the link above, they say they are colloquial and I have seen another one, which I accidentally closed and there it was said that only the -s forms that are adjacent to a verb are considered standard Czech (ie. přišels, viděls, četls, etc.; so "tams" would be wrong like tbiedl said). It's just my curiosity that I'm asking... :-)
we are not ÚJČ, so we generally go by what ÚJČ says. the link you provided provides the justification for our decision rather than tbiedl's challenge. and before we get into it, the translation you are all questioning is a side alternative.
Ve 2. os. j. č. lze příslušný tvar slovesa být (jsi) nahradit pouhým -s
připojovaným k prvnímu přízvučnému slovu ve větě,
nejčastěji jím bývá právě příčestí: Udělal jsi / Udělals, co jsem ti řekl? Byla jsi / Bylas včera doma? Ty jsi / Tys tomu dal! Viděl jsem, že jsi / žes přišel. Podívej, co jsi / cos natropil. Podoby se jsi se považují za základní, neutrální způsob vyjádření.
Podoby se -s jsou hodnoceny jako hovorové.
note the "appended to the first stressed word" language, and compare the "usually it is precisely the participle" with the examples given. there clearly is room afforded to accept attachment to other first stressed words than the verb. and at least the source we both cite throws all of these in the same "colloquial" bucket, although most non-verbs do have a decidedly sloppy feel. (ETA: let's keep in mind that "hovorové" does not equal "non-standard".)
now a practical perspective. consider that "byls" is, in a way, a contracted form of "byl jsi", and if we accepted appending the -s to words in other than first stressed positions, our users would wonder why we are so permissive with the word-enclitic -s and so stringent with the full jsem/jsi/jste. we will simply not accept Tam byl jsi. because this is not a course for budding poets. so let's go for consistency, given the available evidence.
Thanks for the comment. I've asked the above question at the time when I was not sure if Colloquial Czech is accepted throughout this course. On multiple occasions, the contributors said that Colloquial Czech is not accepted (and some of them still do nowadays), while in fact, I found out later that they are all talking about not accepting Common Czech. :-)