This is often the hardest part for Sinhala speakers. A good PDF will use tables to compare these to the articles we use in English or Sinhala.
In addition to textbooks and language courses, there are several PDF resources available for German language learning in Sinhala. Here are a few: german language learning in sinhala pdf
Use these platforms to reinforce the lessons found in your PDFs: This is often the hardest part for Sinhala speakers