What tells us that the subject is 'he'?
Context. Since there is none for this sentence and the verb only determines 3rd person, you can use any of "he, she, it and they" for the translation to be correct.
Prosim lze pouzit He seldom is sleeping here
Ne, pro opakovane akce se pouziva pritomny cas prosty.
"He sleeps here seldomly" . . .this word order was nit accepted. Is it incorrect?
seldomly is not good, seldom is an adverb already
"Zřídka spí tady" is alright too for our owl ?
Yes, but be aware that placing the adverb sentence-final is not as natural as in English. Or rather that the word order changes the meaning, if the adverb contains the new information of the sentence, it comes last.
Is it just me or are there no tipps and notes for adverbs and conjugations?
No, it is not just you