Gloss on words in the passage 'Once or twice a minute all summer' -- Harry Potter Vietnamese translation.

(Note: Vietnamese no longer uses Chinese characters. Characters are given only as an aid to understanding etymology.)

một trong
  'one of'
hai đứa   'two' + classifier of children
sinh đôi   'twin'
tỏ vẻ   'to seem'
ngạc nhiêṇ (愕然) 'surprised'
'One of the two twins looked surprised'
  'you' (literally elder brother)
  'to be, is, are'
huynh trưởng   'prefect' (the translator appears to have created this word, which means 'elder (student) of the school'). E-V dictionaries suggest lớp trưởng as a translation.
hả   sentence-final particle
anh   'elder brother'
Percy   'Percy'
'You are a prefect, elder brother Percy'.
lẽ ra   'Why not'
anh   'you'
phải   'not' (short for không phải). With lẽ ra above means 'you should have done something'.
nói   'say'
chớ   sentence-final particle 'aren't you, didn't you?'
'Why didn't you say?' = 'You should have told us'
tụi em   'we' (younger person talking to older)
chẳng   'not yet'
biết   'know'
gỉ̉   'something, anything'
'We didn't know anything at all.'
có mà   'yes, certainly'
đứa   classifier for children
thứ hai   'second'
sinh đôi   'twin'
nói chen   'put in a word edgewise'
'Yes he did! the second twin interrupted'.
khoan   'slow' or 'just a minute!'
tao   'I' (familiar, used between friends, etc. Also used by superiors to inferiors)
nhớ   'remember'
  'there is' / 'to have'
lần   'time'
ảnh   'he' (Southern dialect) = anh ẩy
  'to have', here has emphatic function ('did say something')
nói   'to say'
gì đó   'of something'
về   'concerning'
vụ   'matter, business, 'affair'
huynh trưởng   'prefect' (see above)
'Just a minute! I remember there was a time he did say something about (the matter of) a prefect'.
hình như   'to seem, appear'
  'there is' / 'to have'
một   'one'
lần   'time'
'Appears there was one time'
hổng chừng   'maybe' (Southern dialect)
hai   'two'
lần   'time'
'Maybe two times'
để   'let me'
nhớ   'to remember'
coi   'to see' = 'try'
'Let me try to remember'
hình như   'to seem, to appear'
nói   'to say'
suốt   'entire'
mùa hè   'summer'
'Seems (he) said (it) all summer'
Anh Percy   'brother Percy'
huynh trưởng   'prefect'
bảo   'to say'
thôi   'stop, cut it out!' (used with regard to discussing a subject).
im   'silence, quite'
nào   sentence final particle (emphatic)
Elder brother Percy Prefect says 'Stop, shutup!'

Vietnamese word order is quite close to English and a word-for-word gloss generally manages to convey the meaning. One problem is the use of sentence-final particles in Vietnamese. These are quite rich and varied in meaning and usage. Whereas in English ending a sentence with a 'huh' or an 'eh' could sound familiar or disrespectful if used inappropriately, Vietnamese has sentence-final particles that indicate respect to the listener. It can be quite difficult representing the nuances of such particles in English.