Why is it "...buji" and not "...buju"? I thought that 1st person singular ends with -uju?
Both are possible, -uji is more formal.
How about Nepotrebuju ni?
The pronoun forms that begin with "n" are used after prepositions.
...except for one of them: "nas". Still a helpful tip; thanks!
Why isn't it "ni" here... Isn't it accusative?
The "n-" forms are used with prepositions.
So could you say "Divám se na ni" for "I am looking at her"? But you'd say "Vidím ji" for "I see her"?
I'm not Czech but as a Slav, I'd say yes, it sounds correct.
I am also a Slav and I am diligently learning Czech, so I would say that it is very correct.
"Ja ji vidim" or "Vidim ji"
Both variants are mainly used, depending on the emphasis.
Is "ji nepotřebuji" correct without já?
Yes, ji is an inconstant clitic.
Is there a synonym for ji that could be used at the last position? Nepotřebuju...
Ji can be also be used in the first and the last position.
Why is ''Nepotřebují ji ''not correct ?
That means "They don't need her".
Mind the difference between "-ji" and "-jí" endings. Their similarity might be a reason why modern Czech prefers using "-ju" in the first person singular.