Erhua is an r-colored ending that can merge with a preceding syllable.
Notice the spelling#
An added r can indicate a suffix rather than an extra full syllable. Compare a dictionary or a speaker recording when learning a particular word.
Suggested practice#
The reference contrasts wán 玩 with wánr 玩儿. Ask a teacher or use a speaker recording to compare the full syllables, rather than adding a separate er syllable mechanically. Usage varies by word, speaker and region; this page does not tell you to add r to every noun.
Reference#
Pronunciation reference. The practice instructions here are newly written. Recordings, charts and illustrations from the reference are not reproduced.
