Why is that "piji" and not "piju"?
The form "piju" is colloquial. You would say it, but not write it.
Both words are grammatically correct and are interchangeable. The only difference is that in colloquial language you will hear more often "piju" while "piji" is more used in literature, proffesional texts and events which require standard language.
Thanks. I presume it's much the same with "piji" and "pijou" for "they are drinking"?
oni pijí (accented I)
Just a detail but worth pointing out..
Czech has only one type of present time so yes, it's same.
Is it not "I am drinking that?" as a translation? or I will drink that?
Can "Ja piji" stand alone?
"I am drinking that" is OK. "I will drink that" is not, this is present tense. "Já piji" is valid and means solely "I drink" or "I'm drinking".