Synonyms that are in the dictionary are marked in green. Synonyms that are not in the dictionary are marked in red.
Antonyms that are in the dictionary are marked in green. Antonyms that are not in the dictionary are marked in red.
If you wish to use the TTS feature for free, you'd have to make do with the robotic-sounding voices.
Source: https://www.slashgear.com/1351264/best-ebook-apps-android/