What this means for learners
TETUN and Tongan; Tonga share 9 sounds — roughly 47% of Tongan; Tonga's inventory overlaps with TETUN. Shared sounds are ones a speaker already knows from their native language and will generally produce and perceive accurately without explicit training.
The 8 sounds found only in Tongan; Tonga represent the greatest pronunciation challenge for TETUN speakers. The adult brain tends to map unfamiliar sounds onto the closest native equivalent — a process that produces the characteristic "accent" of a second-language speaker. Learning to hear and produce these sounds as distinct requires focused ear training, not just repetition.
Conversely, TETUN has 10 sounds not used in Tongan; Tonga. Native Tongan; Tonga speakers learning TETUN will face the mirror-image challenge with these sounds.
Phoneme inventories from PHOIBLE. Data reflects one documented inventory per language; some variation exists across dialects and sources.