Complex sentence: In Dutch, the letter "Y" today only occurs in loanwords, proper names, or in (variantly spelled) old Dutch, while in the related language Afrikaans, "Y" has completely replaced "IJ".
Split into simpler sentences: In Dutch, the letter "Y" today only occurs in loanwords, proper names, or in (variantly spelled) old Dutch. However, in the related language Afrikaans, the "Y" has replaced the "IJ".